JAMB UTME - Chemistry - 2015

Vulcanization involves the removal of

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Vulcanization involves the creation of chemical bonds between rubber molecules by cross-linking them with sulfur or other chemicals. This process helps to increase the strength, durability, and elasticity of the rubber. During vulcanization, the double bonds in the rubber molecules are broken and new cross-links are formed, which results in the formation of a polymer network. Therefore, the answer is the double bond, as it is removed or broken during the process of vulcanization.

The number of isomers formed by C6H14 is

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The number of isomers formed by C6H14 is 5. C6H14 is the molecular formula for hexane, a type of hydrocarbon with six carbon atoms and 14 hydrogen atoms. Isomers are molecules that have the same molecular formula but different structural arrangements of atoms. In the case of hexane, there are several possible structural arrangements that can result in different isomers. These include: - Straight-chain hexane: This is the most common form of hexane, where all six carbon atoms are arranged in a straight line. - 2-Methylpentane: This is an isomer where one of the carbon atoms in the straight-chain hexane is replaced by a methyl (-CH3) group. - 3-Methylpentane: This is an isomer where a different carbon atom in the straight-chain hexane is replaced by a methyl group. - 2,3-Dimethylbutane: This is an isomer where two of the carbon atoms in the hexane molecule have methyl groups attached to them. - 2,2-Dimethylbutane: This is an isomer where three of the carbon atoms in the hexane molecule have methyl groups attached to them. Therefore, there are five isomers formed by C6H14.

According to the kinetic theory, an increase in temperature causes the kinetic energy of particles to

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According to the kinetic theory, an increase in temperature causes the kinetic energy of particles to increase. All matter is made up of tiny particles called atoms and molecules that are constantly in motion. The kinetic energy of these particles is related to their temperature, which is a measure of the average kinetic energy of all the particles in a substance. When you increase the temperature of a substance, you add energy to the particles, causing them to move faster and collide with each other more frequently and with greater force. This increase in collisions and energy results in an overall increase in the kinetic energy of the particles. Therefore, an increase in temperature leads to an increase in the kinetic energy of the particles, while a decrease in temperature leads to a decrease in the kinetic energy of the particles.

The major source of oxides of nitrogen is from the burning of

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The major source of oxides of nitrogen (NOx) is from the burning of fuel, such as gasoline and diesel, in internal combustion engines like those found in cars, trucks, and airplanes. When fuel is burned, it reacts with the air to produce heat and power. However, this reaction also produces NOx, which can contribute to air pollution and have negative impacts on human health and the environment.

PCl5(g) ↔ PCl3(g) + Cl2(g)

In the reaction above, a decrease in pressure will

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In the reaction above, a decrease in pressure will increase the yield of PCl3. This is because pressure and the direction of a reaction are related through Le Chatelier's Principle. According to this principle, if a stress is applied to a system at equilibrium, the system will respond in a way that tends to counteract the stress. In this case, the decrease in pressure is a stress applied to the system, and the system will respond by shifting the equilibrium to the side with the greater number of moles of gas, which is the side with PCl3 and Cl2. This shift will increase the yield of PCl3 and decrease the yield of PCl5.

An element used in the production of matches is

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An element used in the production of matches is sulphur. Sulphur is a chemical element that is commonly found in nature, and it is used in the production of matches because of its flammable properties. The striking surface of a matchbox contains a mixture of sulphur and other chemicals. When the match is struck against the surface, the friction generates heat, which ignites the sulphur and other chemicals. This produces a flame that can light the matchstick, which is made of wood and coated with a mixture of chemicals that also include sulphur. Sulphur is a highly reactive element, which makes it an ideal choice for use in matches. It is also a non-metal, which means it has some unique properties, such as being a good insulator of electricity. However, sulphur is also a toxic substance, and its use in matches has been regulated in some countries to reduce the risk of health and environmental hazards.

Detergents are manufactured with straight hydrocarbon chains so as to make them

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The acid used in electrolysis of water is dilute

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None of the given options are commonly used as the acid in the electrolysis of water. In fact, the most common acid used in the electrolysis of water is dilute sulfuric acid (H2SO4). The purpose of the acid in the electrolysis of water is to increase the conductivity of the water. Pure water is a poor conductor of electricity, but by adding a small amount of acid, it can become a better conductor. This is because the acid dissociates into ions in the water, which allows the electrical current to pass more easily through the solution. However, it's important to note that the acid used in the electrolysis of water is typically only slightly acidic or "dilute." This is because too much acid can cause unwanted side reactions and can also corrode the electrodes used in the process. Therefore, the acid is usually added in a small amount to achieve the desired conductivity without causing any harm to the equipment or the process itself.

In countries where the temperature falls below 273K, salt is always sprinkled on the icy road in order to

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Salt is sprinkled on icy roads in cold countries to lower the melting point of the ice. When salt is spread on the ice, it dissolves and creates a brine solution with a lower freezing point than pure water. This means that the ice will start to melt even when the temperature is below its normal freezing point, making the road safer for vehicles and pedestrians to use. In other words, the salt helps prevent the road from becoming too slippery and dangerous by melting the ice at lower temperatures.

In the electrolysis of CuSO4(g) using platinum electrodes, the reaction at the anode is

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Which of the following types of alkanols undergo oxidation to produce alkanoic acids? 1. Primary alkanols 11. Secondary alkanols 111. Tertiary alkanols

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Only primary alkanols undergo oxidation to produce alkanoic acids. When primary alkanols undergo oxidation, the terminal carbon atom in the alcohol is oxidized to form a carboxylic acid functional group (-COOH) in the corresponding alkanoic acid. Secondary and tertiary alkanols, on the other hand, do not have a terminal carbon atom that can be oxidized to form a carboxylic acid functional group. Therefore, they cannot be oxidized to produce alkanoic acids. In summary, the answer is option 'l only'.

Iron is often galvanized in order to

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Iron is often galvanized in order to protect it against corrosion. Galvanizing involves coating the surface of iron with a layer of zinc. Zinc is more reactive than iron, which means that when the iron is exposed to oxygen and water, the zinc sacrificially corrodes instead of the iron. This protects the iron from rust and other forms of corrosion, which can weaken and damage the metal. The zinc layer also acts as a barrier between the iron and the environment, helping to prevent moisture and other corrosive substances from reaching the iron surface. As a result, galvanized iron is often used in outdoor applications where it is exposed to moisture and other corrosive elements, such as fences, roofs, and gutters.

When sugar is dissolved in tea, the reaction is always accomplished by

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When steam is passed over red-hot carbon, the substances produced are

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When steam is passed over red-hot carbon, the substances produced are hydrogen and carbon (II) oxide. The reaction that occurs is known as the water-gas shift reaction and is represented by the chemical equation: C(s) + H2O(g) → CO(g) + H2(g) In this reaction, steam (H2O) reacts with carbon (C) in the presence of heat to produce carbon (II) oxide (CO) and hydrogen gas (H2). The carbon (II) oxide is formed because the carbon combines with only one of the oxygen atoms from the steam to form CO, while the other oxygen atom combines with hydrogen to form water (H2O). Therefore, the substances produced by passing steam over red-hot carbon are hydrogen gas and carbon (II) oxide.

Chlorine gas turns a damp starch-iodide paper

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Chlorine gas turns a damp starch-iodide paper from its original colour to a dark blue color. The reason behind this change is that chlorine gas reacts with the iodide ions in the paper, oxidizing them to iodine. This iodine then reacts with the starch in the paper, forming a dark blue complex known as starch-iodine complex. Therefore, the presence of chlorine gas can be detected by observing the change in color of the damp starch-iodide paper from its original color to a dark blue color.

Which of the following results in the fall of acid rain?

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The fall of acid rain is primarily caused by the emissions of oxides of sulfur (SOx) and nitrogen (NOx) into the atmosphere. When these gases are released from sources such as factories, cars, and power plants, they can react with other substances in the atmosphere, such as water vapor, to form acidic compounds like sulfuric acid and nitric acid. These acidic compounds then combine with other substances in the atmosphere, such as particulate matter, to form droplets that fall to the ground as acid rain. Particulate matter itself can contribute to air pollution, but it does not directly cause acid rain. Oxides of lead and gaseous hydrocarbons are also air pollutants, but they do not directly contribute to the formation of acid rain. Lead emissions are primarily associated with lead poisoning and other health problems, while gaseous hydrocarbons contribute to the formation of smog and ground-level ozone.

The principal constituent of natural gas is

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The principal constituent of natural gas is methane. Natural gas is a fossil fuel that forms from the remains of plants and animals that lived millions of years ago. It is primarily made up of methane, which is a molecule composed of one carbon atom and four hydrogen atoms. Methane is a colorless, odorless gas that is highly flammable and is the main component of natural gas, typically making up between 70-90% of its composition. Other gases found in natural gas include ethane, propane, and butane, but these are typically present in much smaller quantities. Methane is a potent greenhouse gas, and its release into the atmosphere contributes to climate change.

Hydrogen can be displaced from a hot alkaline solution by

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When a metal is placed into a hot alkaline solution containing hydrogen, the metal can displace the hydrogen from the solution. This is because metals have a higher affinity for electrons than hydrogen does. As a result, the metal will take the electrons from the hydrogen atoms, causing the hydrogen to be released from the solution. In this case, the metal that will displace the hydrogen most effectively is iron (Fe). Copper (Cu), calcium (Ca), and tin (Sn) can also displace hydrogen, but iron is the most effective of the four options.

A suitable reagent for distinguishing between ethanoic acid ethanol is

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A suitable reagent for distinguishing between ethanoic acid and ethanol is ammoniacal silver trioxonitrate (V) or Tollens' reagent. When Tollens' reagent is added to a solution containing ethanol, it does not react, and the solution remains clear. However, when Tollens' reagent is added to a solution containing ethanoic acid, a silver mirror forms on the inside of the test tube. This happens because ethanoic acid is an aldehyde and can be oxidized by Tollens' reagent to produce carboxylic acid and a silver mirror. Ethanol, on the other hand, does not react with Tollens' reagent because it is not an aldehyde. Other reagents like bromine water, Fehling's solution, and sodium hydrogentrioxocarbonate (IV) are not suitable for distinguishing between ethanoic acid and ethanol because they either react with both compounds or do not produce any noticeable change in either of them.

N2O4(g) ↔ 2NO2(g)

In the endothermic reaction above, more product formation will be favoured by

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In the endothermic reaction N2O4(g) ↔ 2NO2(g), more product formation will be favored by a decrease in pressure. This is because the reaction produces two moles of gas for every one mole of N2O4 that reacts. According to Le Chatelier's Principle, when the pressure on a system is decreased, the equilibrium will shift to the side of the reaction with more moles of gas to compensate for the decrease in pressure. In this case, the equilibrium will shift to the side of the reaction that produces more gas molecules, which is the side with more NO2(g) molecules. Therefore, a decrease in pressure will favor the formation of more NO2(g) molecules, resulting in more product formation.

A change in the temperature of a saturated solution disturbs the equilibrium between the

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Which of the following ions require the largest quantity of electricity for discharge at an electrode?

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A liquid that will dissolve fat is

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In the production of soap, concentrated sodium chloride solution is added to

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In the production of soap, concentrated sodium chloride solution is added to decrease the solubility of the soap. Soap is typically made by the saponification reaction between a fat or oil and a strong base, such as sodium hydroxide (NaOH) or potassium hydroxide (KOH). This reaction produces soap molecules, which have both hydrophilic (water-loving) and hydrophobic (water-repelling) ends. During the soap-making process, the soap molecules can remain dissolved in the water layer and not separate out into the soap layer due to their hydrophilic ends, which prefer to interact with water molecules. By adding a concentrated sodium chloride (NaCl) solution, the solubility of the soap molecules in water is decreased due to the formation of ion-dipole interactions between the sodium and chloride ions and the hydrophilic ends of the soap molecules. As a result, the soap molecules are forced to separate out into the soap layer, making them easier to extract and purify. Therefore, the addition of concentrated sodium chloride solution in soap production decreases the solubility of the soap.

In the electrolysis of brine, the anode is

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The anode in the electrolysis of brine is typically made of carbon. During the electrolysis of brine (a solution of sodium chloride), positively charged sodium ions move towards the negatively charged anode, where they receive electrons and are converted into neutral sodium atoms. At the same time, negatively charged chloride ions move towards the positively charged cathode, where they lose electrons and are converted into chlorine gas. The anode material needs to be able to withstand the corrosive environment of the brine and not react with the sodium ions or chlorine gas produced during the electrolysis. Carbon is a commonly used material for the anode because it is stable in this type of environment.

The fuming of kettles is caused by the presence in the water of

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The fuming of kettles is caused by the presence of calcium trioxocarbonate (IV) in the water. This compound is also known as calcium carbonate and is commonly found in hard water. When hard water is heated in a kettle, the calcium carbonate reacts with the heat to form calcium oxide (also known as quicklime) and carbon dioxide gas. The carbon dioxide gas is what causes the fuming or bubbling that you may see coming from the kettle. The calcium oxide also forms a white or grayish powdery substance, known as scale or limescale, which can build up in the kettle and on heating elements over time. This can reduce the efficiency of the kettle and may even cause it to fail over time. Therefore, it is important to descale your kettle regularly if you live in an area with hard water.

the elements that belong to the third period of the periodic table are

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Which of the following gives a precipitate when treated with NaOH solution?

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When NaOH (sodium hydroxide) solution is added to AlCl3 (aluminum chloride), it reacts to form a precipitate, which is a solid that separates out of a solution. The reaction between AlCl3 and NaOH is as follows: AlCl3 + 3NaOH -> Al(OH)3 (precipitate) + 3NaCl (salt) On the other hand, when NaOH solution is added to NH4Cl (ammonium chloride), there is no precipitate formed. Instead, the reaction between NH4Cl and NaOH is as follows: NH4Cl + NaOH -> NH3 (ammonia gas) + NaCl (salt) When NaOH solution is added to CH3COONa (sodium acetate), there is also no precipitate formed. Instead, the reaction between CH3COONa and NaOH is as follows: CH3COONa + NaOH -> CH3COOH (acetic acid) + Na2CO3 (sodium carbonate) Finally, when NaOH solution is added to Na2CO3 (sodium carbonate), there is also no precipitate formed. Instead, the reaction between Na2CO3 and NaOH is as follows: Na2CO3 + NaOH -> Na2CO3 (sodium carbonate) + H2O (water) In summary, only AlCl3 forms a precipitate when treated with NaOH solution.

The energy value of petrol can be determined by a

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If glucose is heated with concentration tetraoxosulphate (VI) acid, it will be dehydrated to

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What is the pH of 0.001 mol dm4 solution of sodium hydroxide?

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The pH of a solution of sodium hydroxide (NaOH) is a measure of its acidity or basicity. The pH scale ranges from 0 to 14, with 7 being neutral, less than 7 being acidic, and greater than 7 being basic. A 0.001 mol/dm^3 solution of NaOH is a very dilute solution, but it is still basic. NaOH is a strong base and dissociates completely in water to form hydroxide ions (OH-), which are responsible for its basicity. The pH of a basic solution can be calculated using the equation: pH = -log[OH^-] where [OH^-] is the concentration of hydroxide ions in the solution. In this case, the concentration of hydroxide ions is equal to the concentration of the NaOH solution, which is 0.001 mol/dm^3. So, pH = -log(0.001) = 3 Therefore, the pH of a 0.001 mol/dm^3 solution of sodium hydroxide is 11.

The substance that is used in the steel industry for the removal of carbon, sulphur and phosphorus impurities from pig iron is

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The substance used in the steel industry for the removal of carbon, sulphur, and phosphorus impurities from pig iron is oxygen. This process is called "oxidation" and it occurs when oxygen reacts with the impurities in the pig iron, which then form oxides that can be easily removed. During the steelmaking process, pig iron is melted in a furnace and then impurities such as carbon, sulphur, and phosphorus are removed. Oxygen is blown into the furnace, which reacts with the impurities to form oxides. These oxides rise to the surface of the molten metal and form a slag layer, which is removed. This process results in the production of steel with lower impurity levels and higher purity. In summary, oxygen is used in the steel industry for the removal of impurities from pig iron through the process of oxidation, which forms oxides that can be easily removed.

The gas that is most useful in protecting humans against solar radiation is

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The gas that is most useful in protecting humans against solar radiation is ozone. Ozone is a molecule that is made up of three oxygen atoms, and it is present in the Earth's atmosphere in the ozone layer, which is located about 10-50 kilometers above the Earth's surface. The ozone layer acts as a shield against the harmful ultraviolet (UV) rays that are emitted by the sun. UV rays can cause skin cancer, cataracts, and other health problems in humans, as well as damage crops and other living organisms. When UV rays hit the ozone molecules in the ozone layer, they are absorbed and converted into heat, which dissipates harmlessly into the atmosphere. This process protects us from the harmful effects of UV rays and helps to maintain a healthy environment on Earth. Therefore, the ozone layer is considered essential for the survival of life on Earth, and its protection is a critical environmental issue.

Electronic configuration of an elements 13x in the subsidiary energy level is

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The electronic configuration of an element 13x in the subsidiary energy level is 1s22s22p63s23p1. The electronic configuration of an element describes the way electrons are arranged in the energy levels or shells around the nucleus. The first energy level can hold up to 2 electrons, the second energy level can hold up to 8 electrons, and the third energy level can hold up to 18 electrons. For element 13x, we can determine its electronic configuration by using the periodic table. Element 13x is in group 13, which means it has 3 valence electrons. The valence electrons are the outermost electrons that are involved in chemical reactions. The electronic configuration of element 13x in the ground state, which means it is not excited, is 1s22s22p63s23p1. This means that the first energy level is full with 2 electrons (1s2), the second energy level is also full with 8 electrons (2s22p6), and there is 1 electron in the third energy level (3s23p1). This electron is a valence electron and is involved in chemical bonding. The other options listed do not have the correct number of electrons in the valence shell, and therefore are not the correct electronic configuration for element 13x.

When a salt loses its water of crystallization to the atmosphere on exposure, the process is said to be

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The correct term is "Efflorescence". Efflorescence is a process where a salt loses its water of crystallization to the atmosphere, causing the salt crystals to dissolve and eventually evaporate, leaving behind a white powdery residue on the surface. This can happen due to changes in temperature, humidity, or air movement. Some examples of salts that can effloresce include sodium carbonate, calcium sulfate, and magnesium chloride. It's important to note that efflorescence is different from the other three options you listed: - Deliquescence refers to a substance that absorbs moisture from the air until it dissolves in the absorbed water. - Effervescence is the release of gas (typically carbon dioxide) from a substance, often in the form of bubbles. - Fluorescence is the property of certain substances to emit light (typically in the ultraviolet or visible spectrum) after absorbing light or other electromagnetic radiation.

Cathode rays causes an object placed behind a perforated anode to cast a shadow on the screen. This observation shows that the rays

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The observation of a shadow cast by an object placed behind a perforated anode when cathode rays are passed through it shows that the rays travel in straight lines. This is because only the areas not blocked by the object can be illuminated on the screen. Therefore, options A, B, and C are incorrect, as none of them are related to the observed phenomenon.

Which of the following metals burn with brick red flame?

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Calcium (Ca) burns with a brick red flame. When a metal burns, it reacts with oxygen in the air to form a metal oxide. The color of the flame depends on the energy released during the reaction, which can vary depending on the metal. Calcium has a relatively low ionization energy, meaning it requires less energy to remove an electron from its outermost shell compared to other metals such as lead (Pb), sodium (Na), and magnesium (Mg). When calcium reacts with oxygen in the air, it releases a relatively large amount of energy, resulting in a bright, brick red flame. In contrast, lead, sodium, and magnesium have higher ionization energies, meaning they require more energy to remove an electron from their outermost shells. As a result, their reactions with oxygen release less energy and produce flames of different colors. In summary, calcium burns with a brick red flame because it has a relatively low ionization energy and releases a large amount of energy when it reacts with oxygen in the air.

The filter in the cigarette reduces the nicotine content by

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The filter in a cigarette does not reduce the nicotine content through burning, evaporation, or absorption. Instead, it works through a process called adsorption. Adsorption is the process where a substance (in this case, nicotine) adheres to the surface of a solid material (in this case, the filter). The filter in a cigarette is made of a porous material that attracts and holds onto the nicotine as it passes through. This reduces the amount of nicotine that is inhaled by the smoker. So, the filter in a cigarette reduces the nicotine content by adsorption.

A compound contains 40.0% carbon 6.7% hydrogen and 53.3% oxygen. If the molar mass of the compound is 180, find the molecular formula.

[H = 1, C = 12, O = 16]

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First, we need to determine the empirical formula of the compound, which gives the simplest whole-number ratio of atoms in the compound. To do this, we assume that we have 100 g of the compound, which means we have 40.0 g of carbon, 6.7 g of hydrogen, and 53.3 g of oxygen. Next, we convert these masses to moles by dividing by the respective atomic or molecular masses: - Carbon: 40.0 g / 12.0 g/mol = 3.33 mol - Hydrogen: 6.7 g / 1.0 g/mol = 6.7 mol - Oxygen: 53.3 g / 16.0 g/mol = 3.33 mol The mole ratio of these elements is approximately 1:2:1, which gives us the empirical formula CH2O. To find the molecular formula, we need to know the molecular mass of the empirical formula. The molecular mass of CH2O is: - Carbon: 1 x 12.0 g/mol = 12.0 g/mol - Hydrogen: 2 x 1.0 g/mol = 2.0 g/mol - Oxygen: 1 x 16.0 g/mol = 16.0 g/mol Total: 30.0 g/mol We are given that the molar mass of the compound is 180 g/mol. To find the molecular formula, we divide the molar mass by the empirical formula mass: 180 g/mol ÷ 30.0 g/mol = 6 The molecular formula is the empirical formula multiplied by the whole number 6: 6 x CH2O = C6H12O6. Therefore, the molecular formula of the compound is C6H12O6.

Tartaric acid is used industrially to

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Tartaric acid is primarily used in the food industry to make baking powder. Baking powder is a leavening agent used in baked goods, such as cakes and bread, to help them rise and become fluffy. Tartaric acid is one of the main components of baking powder, along with sodium bicarbonate (baking soda). When baking powder is added to a batter or dough and exposed to heat, the tartaric acid reacts with the baking soda to produce carbon dioxide gas, which causes the mixture to expand and rise. This creates the fluffy texture that we associate with baked goods. While tartaric acid is not commonly used to remove rust or as a drying agent, it is sometimes used in the wine industry to adjust the acidity of wines and as a flavoring agent in some soft drinks and fruit juices. However, its primary industrial use is in the food industry as a component of baking powder.

A fixed mass of gas has a volume of 92cm3 at 3 oC. When will be its volume at 18 oC if the pressure remains constant

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The volume of a fixed mass of gas at constant pressure is directly proportional to its temperature, according to Charles's Law. Therefore, we can use the formula V1/T1 = V2/T2 to solve this problem, where V1 and T1 are the initial volume and temperature of the gas, respectively, and V2 and T2 are the final volume and temperature of the gas, respectively. We are given that the initial volume V1 is 92cm3 and the initial temperature T1 is 3 oC, which we need to convert to the absolute temperature scale (Kelvin). To do this, we simply add 273 to the Celsius temperature to get T1 = 276 K. We are also given that the pressure remains constant, which means that the gas is isobaric. Therefore, we can use the formula V1/T1 = V2/T2 to find the final volume V2. Rearranging the formula, we get: V2 = (V1/T1) x T2 Substituting the values we have, we get: V2 = (92/276) x (18+273) V2 = (1/3) x 291 V2 = 97.0 cm3 Therefore, the volume of the gas at 18 oC will be 97.0 cm3.

The oxidation state of oxygen in tetraoxosulphate(IV) acid is

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The oxidation state of an atom in a compound is the charge it would have if all the shared electrons were assigned to the more electronegative atom. In tetraoxosulphate(IV) acid, which is also known as sulfuric acid, the formula is H2SO4. The oxidation state of hydrogen is +1 because it usually has a +1 oxidation state in compounds. To find the oxidation state of sulfur, we use the fact that the oxidation states of all the atoms in a compound add up to zero. Since there are two hydrogen atoms with a total oxidation state of +2, and four oxygen atoms with a total oxidation state of -8 (since the usual oxidation state of oxygen in a compound is -2), the oxidation state of sulfur must be +6 for the sum of the oxidation states to equal zero. Therefore, the oxidation state of oxygen in tetraoxosulphate(IV) acid is -2, since the sum of the oxidation states of all the atoms in the compound must equal zero.

The leachate of a certain plant ash is used in local soap making because it contains

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In the industrial production of H2 is removed by (solution)

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Washing under pressure is an action in the industrial production of H. Options B,C,D will not yield Co and H

Chlorine, consisting of two isotopes of mass numbers 35 and 37, has an atomic mass of 35.5.

The relative abundance of the isotope of mass number 37 is?

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A difference between chemical and physical change is that in a chemical change

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The main difference between a chemical change and a physical change is that in a chemical change, a new substance is formed. During a chemical change, the atoms or molecules of the original substance rearrange themselves to form a new substance with different properties. In contrast, a physical change involves a change in the physical state or appearance of a substance, but the chemical composition of the substance remains the same. For example, melting ice is a physical change because the solid water (ice) is converted into liquid water, but the chemical composition of the water remains the same. Therefore, the correct answer to the question is that in a chemical change, a new substance is formed. The other options may be associated with chemical changes as well, but they are not defining characteristics of chemical changes.

Which of the following compounds can be represented by the molecular formula C2H6O?

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Ethanol can be represented by the molecular formula C2H6O. The molecular formula of a compound gives you the number of atoms of each element that are present in one molecule of the compound. In this case, the molecular formula C2H6O tells us that there are two carbon atoms, six hydrogen atoms, and one oxygen atom in one molecule of the compound. Ethanol is a common alcohol that is used in many products, including alcoholic beverages, fuel, and solvents. Its molecular formula is C2H6O, which matches the formula given in the question. Propane (not Propanel) has the molecular formula C3H8, methanoic acid has the molecular formula HCOOH, and glucose has the molecular formula C6H12O6. Therefore, the only compound among the options that matches the molecular formula C2H6O is ethanol.

NH3(g) + HCl(s) → NH4Cl(s)

The entropy change in the system above is

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The weakest attractive force that can be observed between two molecules is

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The weakest attractive force that can be observed between two molecules is called the van der Waals force. Van der Waals forces are relatively weak attractive forces that exist between molecules due to the temporary fluctuation in the electron density within the molecules. These fluctuations create temporary dipoles that can attract other molecules with opposite charges. Although van der Waals forces are relatively weak compared to ionic or covalent bonds, they play an important role in determining the physical and chemical properties of substances. For example, the boiling point of a substance is determined by the strength of the intermolecular forces between its molecules. Molecules with stronger van der Waals forces have higher boiling points than molecules with weaker van der Waals forces.

Alloys are best prepared by

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Alloys are best prepared by cooling a molten mixture of the metals. This is because when metals are melted, they become more reactive and are able to mix and combine evenly. By combining different metals in a molten state, they can form a new material with unique properties that are different from the original metals. This process of cooling the molten mixture of metals is called solidification, which allows the metals to solidify into a homogeneous mixture, forming an alloy. This method is widely used in industry and can be used to create alloys with precise compositions and properties for specific applications. Electroplating involves coating a metal with a thin layer of another metal, which is different from forming an alloy. Arc-welding, on the other hand, is used to join metals together, but it doesn't create a new material with different properties. Reducing and mixing metallic oxides is another method used to form alloys, but it is less common and more complicated than the process of melting and cooling a mixture of metals.