WAEC SSCE - Chemistry - 2018

Détails de la réponse

One of the criteria for confirming the purity of benzene is to determine its boiling point. The boiling point of a substance is a physical property that is determined by the intermolecular forces between the particles in the substance. These forces are dependent on the molecular structure of the substance, and so the boiling point is unique for each substance. In the case of benzene, it has a known and well-defined boiling point of 80.1 °C. By determining the boiling point of a sample and comparing it to the known boiling point of pure benzene, we can determine the purity of the sample. If the boiling point of the sample is different from the known boiling point of pure benzene, it indicates that the sample is impure and contains other substances. So, determining the boiling point of a sample of benzene is a useful way to confirm its purity.

Détails de la réponse

Out of the given compounds, only NaH (sodium hydride) contains ionic bonds involving hydrogen. In NaH, hydrogen has a negative charge (H\(^-\)) and sodium has a positive charge (Na\(^+\)). This results in the formation of an ionic compound. In the other compounds, hydrogen forms covalent bonds. In CH\(_4\) (methane), hydrogen shares electrons with carbon to form four covalent bonds. In HCl (hydrogen chloride), hydrogen shares electrons with chlorine to form a single covalent bond. In NH\(_3\) (ammonia), hydrogen shares electrons with nitrogen to form three covalent bonds. Therefore, only NaH contains ionic bonds involving hydrogen, while the others have covalent bonds.

Détails de la réponse

Détails de la réponse

To determine the empirical formula of the compound, we need to find the simplest whole number ratio of the atoms present in the compound. Given that the compound contains 0.067 mol Cu and 0.066 mol O, we need to first convert the moles to whole numbers. Dividing both values by the smaller value (0.066 mol), we get: Cu: 0.067 mol ÷ 0.066 mol ≈ 1.02 ≈ 1 O: 0.066 mol ÷ 0.066 mol = 1 So, the ratio of Cu to O atoms is 1:1. Therefore, the empirical formula of the compound is CuO. We can check that the formula is correct by calculating the molar mass of CuO and comparing it to the experimental data. Molar mass of CuO = 63.5 g/mol + 16 g/mol = 79.5 g/mol The experimental data gives us: (0.067 mol)(63.5 g/mol) + (0.066 mol)(16 g/mol) = 4.2495 g The molar mass calculated from the experimental data is: (4.2495 g) / (0.133 mol) ≈ 31.97 g/mol This is close to the molar mass of CuO (79.5 g/mol), which confirms that the empirical formula is CuO.

Détails de la réponse

The electron configuration for Chromium \(_{24}Cr\) is \(1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1} 3d^{5}\). Electron configuration is a description of the arrangement of electrons in an atom. It is represented by a series of numbers and letters that indicate the energy level, or shell, and the orbital, or subshell, that each electron resides in. In the case of Chromium, the electron configuration starts with the first energy level, which is filled with two electrons in the 1s orbital. The second energy level has two electrons in the 2s orbital and six electrons in the 2p orbital. The third energy level has two electrons in the 3s orbital and six electrons in the 3p orbital. Finally, the fourth energy level has one electron in the 4s orbital and five electrons in the 3d orbital. So, out of the given options, the electron configuration that represents Chromium \(_{24}Cr\) is \(1s^{2} 2s^{2} 2p^{6} 3s^{2} 3p^{6} 4s^{1} 3d^{5}\).

Détails de la réponse

In the given reaction, the bromine molecule (Br2) is reacting with two potassium iodide molecules (2KI) to form two potassium bromide molecules (2KBr) and one iodine molecule (I2). During this reaction, the bromine molecule is gaining electrons (receiving electrons) and being reduced, while the iodine molecule is losing electrons (donating electrons) and being oxidized. Therefore, we can conclude that the bromine molecule is acting as a oxidizing agent because it is causing the oxidation of the iodine molecule by accepting electrons, and therefore, itself being reduced.

Détails de la réponse

The correct answer is that atoms are electrically neutral because they contain an equal number of protons and electrons. Protons have a positive charge, while electrons have a negative charge. In an atom, the number of protons in the nucleus is equal to the number of electrons orbiting around the nucleus. The positive charge of the protons is balanced by the negative charge of the electrons, resulting in an overall charge of zero for the atom. This balance of positive and negative charges is what makes atoms electrically neutral.

Détails de la réponse

The law of definite proportions states that pure samples of the same compound contain the same elements combined in the same proportion by mass. This means that no matter where or how the compound is obtained, the ratio of the masses of the elements in the compound will always be the same. For example, if a water molecule contains 2 hydrogen atoms and 1 oxygen atom, the ratio of the masses of hydrogen to oxygen will always be 2:1 in any sample of water.

Détails de la réponse

Détails de la réponse

The method that can be used to separate blood cells from plasma is: Centrifugation. Centrifugation is a process of separating components in a mixture based on their density differences. In the case of blood, when it is spun in a centrifuge, the heavier components, such as the blood cells, are forced to the bottom of the tube while the lighter components, such as plasma, stay at the top. The separation is based on the fact that blood cells are denser than plasma. Therefore, centrifugation is the method that can be used to separate blood cells from plasma. Filtration, chromatography, and distillation are other methods of separating mixtures, but they are not suitable for separating blood cells from plasma. Filtration is the process of separating solids from liquids using a filter. Chromatography is a laboratory technique for separating mixtures into their individual components. Distillation is the process of separating components of a mixture based on differences in their boiling points.

Détails de la réponse

Benzene is the organic compound that can undergo both addition and substitution reactions. Benzene is an aromatic compound with a unique structure that includes a ring of six carbon atoms, each of which is bonded to a hydrogen atom. The electrons in the carbon-carbon double bonds are delocalized, making the molecule more stable and less reactive than typical alkenes. However, the delocalized electrons can still participate in reactions, including addition and substitution. Addition reactions involve the breaking of a double or triple bond and the addition of new atoms or groups of atoms. Because benzene already has delocalized double bonds, it can undergo addition reactions such as hydrogenation and halogenation, in which hydrogen or a halogen is added to the ring. Substitution reactions involve the replacement of one atom or group of atoms with another atom or group of atoms. Benzene is particularly prone to substitution reactions because the delocalized electrons in the ring make it more susceptible to attacks by electrophiles, which are electron-deficient species that can accept a pair of electrons. Substitution reactions include nitration, sulfonation, and halogenation.

Détails de la réponse

Détails de la réponse

Copper (\(Cu\)) and nitric acid (\(HNO_{3}\)) are not suitable for preparing hydrogen gas because of their reactivity and oxidation properties, respectively. When copper is placed in nitric acid, a redox reaction occurs, where the copper is oxidized by the nitric acid. This reaction produces copper nitrate and nitrogen dioxide gas, but it does not produce hydrogen gas. Similarly, nitric acid is a strong oxidizing agent and can oxidize hydrogen gas to water, thereby preventing the formation of hydrogen gas. Therefore, neither copper nor nitric acid can be used to prepare hydrogen gas. Other metals, such as zinc or magnesium, can be used to react with acids to produce hydrogen gas because they are more reactive than copper and do not have the same oxidation properties as nitric acid.

Détails de la réponse

Détails de la réponse

The position of equilibrium in a reversible reaction is affected by the change in concentration of the reactants. When the concentration of reactants changes, the rate of the forward and reverse reactions will also change, which will shift the position of the equilibrium to counteract the change in concentration. For example, if the concentration of reactants increases, the rate of the forward reaction will increase, which will shift the position of the equilibrium to the right to counteract the increase in concentration. Conversely, if the concentration of reactants decreases, the rate of the reverse reaction will increase, which will shift the position of the equilibrium to the left to counteract the decrease in concentration.

Détails de la réponse

In the given chemical equation, the reactants are iron(III) chloride (\(FeCl_{3}\)) and hydrogen sulfide (\(H_{2}S\)), and the products are iron(II) chloride (\(FeCl_{2}\)), hydrogen chloride (\(HCl\)), and sulfur (\(S\)). Let's consider the oxidation state of iron in the reactants and products. Iron has a variable oxidation state, and it can exist in either +2 or +3 oxidation state in its compounds. In \(FeCl_{3}\), iron has an oxidation state of +3. Hydrogen sulfide (\(H_{2}S\)) has sulfur in -2 oxidation state. In the product \(FeCl_{2}\), iron has an oxidation state of +2. Sulfur in the product is in 0 oxidation state. Therefore, in the given chemical equation, the oxidation state of iron changes from +3 in the reactant (\(FeCl_{3}\)) to +2 in the product (\(FeCl_{2}\)). Hence the correct answer is "+3 to +2".

Détails de la réponse

Détails de la réponse

Détails de la réponse

Détails de la réponse

Particles in a solid exhibit vibrational motion. In a solid, the particles are closely packed together and held in a fixed position by strong intermolecular forces. However, the particles are not completely motionless. They can vibrate around their fixed positions due to thermal energy. The amount of energy available for this vibrational motion is determined by the temperature of the solid. As the temperature increases, the particles vibrate more vigorously. However, the particles do not move from their positions in the solid, and so they do not exhibit translational motion. Therefore, the correct answer is that particles in a solid exhibit vibrational motion. The other options (vibrational and translational motion, vibrational and random motion, and random and translational motion) are not correct because they describe the motion of particles in liquids or gases, where the particles are free to move and exhibit translational or random motion in addition to vibrational motion.

Détails de la réponse

The correct answer is that chlorine is more electronegative than hydrogen. Electronegativity is a measure of an atom's ability to attract electrons towards itself in a covalent bond. Chlorine has a higher electronegativity than hydrogen, which means that the bonding pair of electrons in a hydrogen chloride molecule is pulled more towards the chlorine atom than towards the hydrogen atom. This creates a partial positive charge on the hydrogen atom and a partial negative charge on the chlorine atom, forming a polar molecule.

Détails de la réponse

Ethene is produced from ethanol by: Dehydration. Dehydration is the process of removing water from a compound. In the case of ethanol, the process of dehydration involves removing a molecule of water (H2O) from two molecules of ethanol (C2H5OH) to form one molecule of ethene (C2H4). The reaction is typically carried out using a catalyst, such as concentrated sulfuric acid or aluminum oxide. The chemical equation for the reaction of ethanol to produce ethene by dehydration is: C2H5OH → C2H4 + H2O Therefore, the correct answer is "dehydration" - ethanol is converted to ethene by removing a molecule of water through the process of dehydration. The other options - decomposition, hydrolysis, and ozonolysis - do not accurately describe the process of producing ethene from ethanol.

Détails de la réponse

Détails de la réponse

The ratio of carbon to hydrogen in the hydrocarbon can be determined using the given mass ratio of carbon and the fact that the molecular weight of the hydrocarbon is the sum of the atomic weights of its constituent elements. Let's assume a sample of 1 gram of the hydrocarbon, which contains 0.93 grams of carbon. This means that the mass of hydrogen in the sample is 1.0 - 0.93 = 0.07 grams. The number of moles of carbon in the sample can be calculated as: 0.93 g C x (1 mol C / 12.0 g C) = 0.0775 mol C Similarly, the number of moles of hydrogen in the sample can be calculated as: 0.07 g H x (1 mol H / 1.0 g H) = 0.07 mol H To find the mole ratio of carbon to hydrogen, we need to divide the number of moles of carbon by the number of moles of hydrogen: 0.0775 mol C / 0.07 mol H = 1.107 Rounding this to the nearest whole number, we get the mole ratio of carbon to hydrogen as 1:1. Therefore, the correct answer is 1:1.

Détails de la réponse

n = cv where n = no of mole, c = molar concentration(mol/dm3) and v = volume( dm3)

volume =  500

cm3
$𝑐$ $𝑐$ ${𝑚}^{}$ $3^{}$ = 0.5dm3, c = 0.005moldm−3
$𝑚$ $𝑚$ $𝑜$ $𝑙$ $𝑑$ ${𝑚}^{}$ ${-}^{}$ $3^{}$massmolarmass
$\frac{𝑚}{}$ $\frac{𝑚}{}$ $\frac{𝑎}{}$ $\frac{𝑠}{}$ $\frac{𝑠}{}$ $\frac{𝑚}{}$ $\frac{𝑜}{}$ $\frac{𝑙}{}$ $\frac{𝑎}{}$ $\frac{𝑟}{}$ $\frac{𝑚}{}$ $\frac{𝑎}{}$ $\frac{𝑠}{}$ $\frac{𝑠}{}$ =  0.0025 = mass98
$\frac{𝑚}{}$ $\frac{𝑚}{}$ $\frac{𝑎}{}$ $\frac{𝑠}{}$ $\frac{𝑠}{}$ $\frac{98}{}$ (where 98g/mol is the molar mass of H2SO4)

number of mole(n) = c x v = 0.5 X 0.005 = 0.0025mol.

but n = 

mass of H2SO4 = 0.0025 x 98 = 0.245g.

Détails de la réponse

Détails de la réponse

The most suitable substance for putting out a petrol fire is not water, carbon(IV) oxide, or a fire blanket, but sand. Sand can be used to smother a petrol fire by covering the flames and cutting off the oxygen supply, which will cause the fire to go out. However, it is important to note that sand should only be used on small fires and not on larger fires, as it can be difficult to spread a large amount of sand over the fire and it may not be effective. In such cases, it is best to evacuate the area and call the fire department.

Détails de la réponse

The property that would not influence electrovalent bond formation is catalytic ability. An electrovalent bond, also known as an ionic bond, is formed when atoms transfer electrons from one to another, resulting in the formation of ions with opposite charges. This bond is typically formed between atoms with significantly different electronegativities. The electronegativity of an atom is a measure of its ability to attract electrons towards itself, and it is a key factor in determining the type of bond that will form between two atoms. Electron affinity is the energy change that occurs when an electron is added to a neutral atom to form a negative ion. This property also influences the formation of electrovalent bonds. Ionization potential is the energy required to remove an electron from an atom. This property can also affect the formation of electrovalent bonds, as it can influence the ability of an atom to give up electrons. Catalytic ability, on the other hand, is the ability of a substance to speed up a chemical reaction without being consumed in the reaction. This property has no direct influence on the formation of electrovalent bonds. To summarize, the property that would not influence electrovalent bond formation is catalytic ability, while electronegativity, electron affinity, and ionization potential all have an impact on the formation of these types of bonds.

Détails de la réponse

Ionic radius is the measure of the size of an ion. It refers to the distance from the center of the nucleus to the outermost electron shell of an ion. When an atom loses or gains electrons to form an ion, its size changes. The trend in ionic radius is that it decreases as the nuclear charge increases. This is because as the nuclear charge increases, the number of protons in the nucleus increases, which leads to stronger attraction between the electrons and the nucleus. This stronger attraction pulls the electrons closer to the nucleus, reducing the size of the ion. Therefore, the correct answer is "decreases as nuclear charge increases".

Détails de la réponse

When chlorine gas is passed through water, it reacts with water to form hydrochloric acid (HCl) and hypochlorous acid (HClO). Hypochlorous acid is a weak acid and can dissociate to form hydrogen ions (H+) and hypochlorite ions (ClO-). The formation of H+ ions in the water will increase the concentration of H+ ions, resulting in a decrease in pH. Therefore, the pH of the water sample will be less than 7 (i.e., acidic) when chlorine is passed through it. It is important to note that the pH of the water sample will depend on the amount of chlorine passed through it, the initial pH of the water, and other factors such as temperature and the presence of other chemicals.

Détails de la réponse

The correct statement about the diagram is: "Energy is absorbed when the ice changes to water." When ice changes to water, it undergoes a physical process known as "melting." During this process, energy is absorbed from the surroundings in order to overcome the attractive forces between the ice molecules and break them apart into individual water molecules. This energy is in the form of heat, and it is absorbed from the surroundings, which results in a decrease in temperature. Therefore, the water in the conical flask will be at a lower temperature than the ice after the ice starts melting. The water molecules do vibrate about a fixed point, but this statement is not relevant to the question being asked.

Détails de la réponse

The process of extracting iron from its ore involves the reduction of the iron oxide (Fe2O3) in the ore to iron metal. This is achieved through a chemical reaction between the iron oxide and carbon (in the form of coke) in a blast furnace. First, the iron ore is crushed and mixed with coke and limestone. The coke, which is mostly carbon, serves as a reducing agent by combining with the oxygen in the iron oxide to form carbon dioxide (CO2). This reaction produces heat and carbon monoxide (CO). The carbon monoxide then reacts with the iron oxide to produce iron metal and more carbon dioxide. The limestone is added to the furnace to remove impurities such as silica (SiO2) that would otherwise interfere with the chemical reaction. The molten iron is then tapped off from the bottom of the furnace and can be further processed into steel.

Détails de la réponse

The valence electrons of magnesium, which has an atomic number of 12, are located in the 3s orbital. In an atom, electrons are arranged in shells surrounding the nucleus. The innermost shell, the 1s orbital, holds the electrons closest to the nucleus and is usually filled first. The next shell, the 2s orbital, is further away from the nucleus and is also filled before the third shell, the 3s orbital. In the case of magnesium, it has 12 electrons, and so the first two shells are completely filled. The 3rd shell, which is the valence shell, has only two electrons in the 3s orbital. These are the valence electrons, and they are the electrons that are involved in chemical reactions and bonding with other atoms.

Détails de la réponse

Détails de la réponse

Détails de la réponse

An acidic salt has hydrogen ions in its aqueous solution. When a salt is formed from a weak acid and a strong base, the resulting salt can still contain some of the acidic properties of the original acid. This means that when the salt is dissolved in water, it can release hydrogen ions (H+) into the solution, making it acidic. This is why the salt is called an acidic salt. In contrast, basic salts contain hydroxide ions (OH-) in their aqueous solution.

Détails de la réponse

To determine the number of atoms in 0.2 moles of nitrogen, we can use Avogadro's number, which represents the number of particles in one mole of a substance. Avogadro's number is equal to 6.02 x 10^23 particles per mole. First, we need to calculate the number of nitrogen atoms in 0.2 moles of nitrogen. We can do this by multiplying the number of moles by Avogadro's number: 0.2 moles x 6.02 x 10^23 particles/mole = 1.204 x 10^23 particles Therefore, there are 1.204 x 10^23 atoms in 0.2 moles of nitrogen. The answer is \(1.20 \times 10^{23}\).

Détails de la réponse

An endothermic reaction is a reaction that requires energy to be absorbed from the surroundings. This means that the reaction absorbs heat and as a result, the surroundings feel cooler. Therefore, the correct option is "reaction vessel feels cool during the reaction". The other options do not necessarily define an endothermic reaction.

Détails de la réponse

Détails de la réponse

In the plastic industry, ethene (also known as ethylene) is commonly used as a raw material. Ethene is a colorless and flammable gas that can be polymerized to produce a wide range of plastics, including polyethylene, which is one of the most widely used plastics in the world. Methane, sulfur, and hydrogen are not typically used as raw materials in the plastic industry.

Détails de la réponse

Détails de la réponse

Détails de la réponse

The empirical formula of the organic compound is given as \(CH_{2}\), which means that the ratio of carbon to hydrogen atoms in the compound is 1:2. The molar mass of the compound is also given as 42\(gmol^{-1}\). To find the molecular formula, we need to determine the actual number of carbon and hydrogen atoms in the compound. Let's assume that the molecular formula of the compound is \(C_{x}H_{y}\). The molar mass of the compound can be calculated as follows: molar mass = (molar mass of carbon) x (number of carbon atoms) + (molar mass of hydrogen) x (number of hydrogen atoms) 42 = (12.0 g/mol) x (x) + (1.0 g/mol) x (y) Simplifying the equation, we get: 12x + y = 42 However, we also know that the ratio of carbon to hydrogen atoms in the compound is 1:2. This means that: x/y = 1/2 Multiplying both sides by y, we get: x = (1/2)y Substituting this value of x in the previous equation, we get: 12(1/2)y + y = 42 Simplifying the equation, we get: 18y = 42 y = 2.33 Since the number of atoms must be a whole number, we can round y to the nearest whole number, which is 2. This means that the molecular formula of the compound is \(C_{x}H_{2}\). Using the ratio of carbon to hydrogen atoms in the empirical formula, we know that x = 2. Therefore, the molecular formula of the compound is \(C_{2}H_{4}\). Hence, the answer is option A) \(C_{2}H_{4}\).

Détails de la réponse

The best explanation for the difference between a gas and a vapor is: Unlike gases, vapors can easily be condensed into liquids. Gases and vapors are both in the gaseous state of matter, which means they have no fixed shape or volume and can expand to fill any container. However, vapors are substances that are normally in the liquid or solid state at room temperature and pressure, but are volatilized (turned into a gas) through evaporation or boiling. The main difference between gases and vapors is that gases are typically elements or compounds that are naturally in the gaseous state, whereas vapors are substances that have been volatilized from their liquid or solid state. Since vapors have been volatilized from their liquid or solid state, they can easily be condensed back into a liquid or solid form by cooling or compressing them. Therefore, the correct statement that explains the difference between a gas and a vapor is that "unlike gases, vapors can easily be condensed into liquids." The other options given are not true or do not accurately explain the difference between gases and vapors.

Détails de la réponse

Environmental pollution can be caused by a variety of factors, including the release of harmful chemicals and substances into the air, water, or soil. Of the options provided, photosynthesis is the factor that would not contribute to environmental pollution. Photosynthesis is the process by which plants use sunlight to convert carbon dioxide and water into oxygen and glucose. This natural process does not release any harmful chemicals or substances into the environment and, in fact, helps to improve air quality by producing oxygen. On the other hand, the production of ammonia, manufacture of cement, and combustion are all activities that can release harmful pollutants into the environment. Ammonia production can release nitrogen oxide and other harmful chemicals into the air and water, while cement manufacturing can release large amounts of carbon dioxide into the atmosphere. Combustion, whether in vehicles or power plants, can release harmful gases and particulate matter into the air. Therefore, photosynthesis is the only option among the given factors that would not contribute to environmental pollution.

Détails de la réponse

Détails de la réponse

The preferential discharge of ions during electrolysis is influenced by the nature of the electrode and electrolytic reactions. During electrolysis, the positive and negative ions in the electrolyte solution move towards the respective electrodes. The electrode's surface and the type of electrolytic reaction determine the preferential discharge of ions. When an electrode's surface has a high affinity for a particular ion, that ion will preferentially discharge at that electrode. For example, if the electrode is made of copper and the electrolyte solution contains copper ions and hydrogen ions, copper ions will preferentially discharge at the electrode's surface. Additionally, the type of electrolytic reaction can influence ion preference. For instance, in the electrolysis of water, hydrogen ions are preferentially discharged at the cathode, and oxygen ions are preferentially discharged at the anode. The mechanism of electrolysis and type of electrolytic cell can also play a role in the preferential discharge of ions.

Détails de la réponse

Détails de la réponse

Détails de la réponse

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None

Détails de la réponse

None