WAEC SSCE - Chemistry - 1989

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Cracking is a process in which large hydrocarbon molecules are broken down into smaller molecules. This is usually achieved by heating the hydrocarbon molecules to high temperatures and then passing them over a catalyst. Cracking is an important process in the petroleum industry because it allows for the production of smaller, more useful hydrocarbons, such as gasoline and diesel fuel, from larger, less useful ones, such as heavy oils and waxes. The smaller hydrocarbons produced by cracking have higher commercial value and are in greater demand for use as fuels, solvents, and raw materials for the production of other chemicals. Therefore, the answer is in which large hydrocarbon molecules are broken into smaller units.

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The oxidation number of an atom is the charge it would have if electrons were transferred completely to the more electronegative atom. Chlorine is more electronegative than most other elements, and thus has a high tendency to gain electrons. The oxidation number of chlorine in KCIO₃ is +5, in CI₂O₇ is +7, in ZnCI₂ is -1, and in HCI is -1. Therefore, the only option where the oxidation number of chlorine is +1 is NaCIO.

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The major raw material in a plastic industry is ethene. Ethene, also known as ethylene, is a colorless gas that is derived from natural gas or petroleum. It is the most commonly used monomer, or building block, in the production of plastic. Ethene is polymerized to form polyethylene, which is used in a wide variety of plastic products, including packaging materials, toys, and household items. The production of plastic from ethene is a highly energy-intensive process and contributes to the emission of greenhouse gases, which can have negative impacts on the environment and climate change.

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The electron configuration of X is 1s² 2s² 2p⁶ 3s¹ which indicates that X has one valence electron in its outermost shell. The valence electron of X is in the 3s subshell, which means that X is in period 3 of the periodic table. The electron configuration of Y is 1s² 2s² 2p⁶ 3s² 3p⁶ 4s², indicating that Y has two valence electrons in its outermost shell. The valence electrons of Y are in the 4s subshell, which means that Y is in period 4 of the periodic table. Therefore, X belongs to group 1, and Y belongs to period 4, which means that the correct option is: "x belongs to group 1, Y belongs to period 4."

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The solubility of a substance is the maximum amount of solute that can be dissolved in a given amount of solvent at a specific temperature. In this question, we are given the volume and concentration of a saturated solution of potassium trioxonitrate (V) at 40°C. We can use this information to calculate the solubility of the salt at 40°C. First, we need to convert the given volume from cm³ to dm³ (litres) since the concentration is given in mol dm^-3. 50 cm³ = 50 ÷ 1000 dm³ = 0.05 dm³ The number of moles of potassium trioxonitrate (V) in the solution can be calculated using the formula: moles = concentration × volume The concentration is given as saturated, which means that the solution contains the maximum amount of potassium trioxonitrate (V) that can dissolve at 40°C. Therefore, the concentration is equal to the solubility of the salt at that temperature. So, we have: moles of KNO₃ = solubility × volume = solubility × 0.05 dm³ We are also given the mass of KNO₃ in the solution, which is 5.05 g. We can use the molar mass of KNO₃ (101 g/mol) to calculate the number of moles: moles of KNO₃ = mass ÷ molar mass = 5.05 ÷ 101 = 0.05 mol Now, we can equate the two expressions for the number of moles of KNO₃: solubility × 0.05 = 0.05 solubility = 0.05 ÷ 0.05 = 1 mol dm^-3 Therefore, the solubility of potassium trioxonitrate (V) at 40°C is 1.0 mol dm^-3. Answer: 1.0 mol dm^-3.

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The molar mass of magnesium is 24.0 g/mol, which means that one mole of magnesium has a mass of 24.0 g. To find the number of atoms in 24.0 g of magnesium, we need to first determine the number of moles of magnesium in 24.0 g. We can do this using the formula: moles = mass/molar mass moles of magnesium = 24.0 g / 24.0 g/mol moles of magnesium = 1.0 mol So, there is 1.0 mole of magnesium in 24.0 g. We can use Avogadro's number (6.02 x 10^23) to find the number of atoms in 1.0 mole of magnesium: number of atoms = Avogadro's number x moles of magnesium number of atoms = 6.02 x 10^23 x 1.0 number of atoms = 6.02 x 10^23 Therefore, there are 6.02 x 10^23 atoms in 24.0 g of magnesium. The answer is 1 x 6.02 x 10^23.

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Cellulose does not usually occur in the crystalline form. Cellulose is a complex carbohydrate that forms long, linear chains of glucose molecules, which are linked by beta-glycosidic bonds. The chains form bundles, which are stabilized by hydrogen bonds. These bundles further form microfibrils, which make up the structural material of plants. Although cellulose has a regular and repeating structure, it does not form crystals because of its complex and branched structure. In contrast, glucose and sucrose can form crystals, and maltose and fructose can occur in both crystalline and amorphous forms.

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The most suitable method to use when separating an insoluble solid from a liquid is filtration. Filtration is a process of separating a solid from a liquid by passing the mixture through a porous material, such as filter paper or a sand bed. The solid is trapped on the filter paper, while the liquid passes through and is collected in a separate container. This method is effective for separating a solid that does not dissolve in the liquid, such as sand from water. Evaporation, magnetization, sublimation, and distillation are methods used for separating a solid from a liquid when the solid is either soluble in the liquid or has a different boiling point than the liquid.

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The condition for a spontaneous reaction is that the Gibbs free energy change (∆G) of the system should be negative. ∆G is related to enthalpy change (∆H), entropy change (∆S), and temperature (T) through the equation: ∆G = ∆H - T∆S For a spontaneous reaction, ∆G must be negative, which can only occur when one of the following conditions is met: - ∆H is negative (exothermic reaction) and ∆S is positive (increase in disorder) - ∆H is positive (endothermic reaction) and ∆S is negative (decrease in disorder), but T∆S is larger than ∆H, so the overall ∆G is negative. Therefore, the option that represents a condition for a spontaneous reaction is: ∆H-T∆S is negative.

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The reaction that represents the laboratory preparation of hydrogen is Zn(s) + 2HCl(aq) → ZnCl₂(aq) + H₂(g). In this reaction, zinc metal reacts with hydrochloric acid to produce hydrogen gas and zinc chloride. The other reactions do not produce hydrogen gas as one of their products or involve hydrogen gas as a reactant.

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Silver chloride is used extensively in manufacturing photographic chemicals. It is a light-sensitive compound that is used in making photographic paper and film. When exposed to light, it undergoes a chemical reaction that results in the formation of a visible image. Silver chloride is also used in making emulsions and toners for black and white photography.

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Locally produced soap in which wood ash is used as the base is usually soft because the wood ash contains a lot of potassium ions. Potassium hydroxide, also known as caustic potash, is formed when wood ash is mixed with water. It is a strong alkali that reacts with fats and oils in the soap-making process to form soap molecules. Potassium ions are responsible for making the soap soft because they have a weaker attraction to water molecules than sodium ions, which are used in the production of hard soap. Therefore, soap made from potassium ions tends to be softer and more soluble in water.

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Copper can best be purified by electrolyzing a copper (ll) salt solution using the impure copper as the anode. Electrolysis is a process in which an electric current is passed through a solution containing ions, causing them to move towards the electrodes. In this case, the impure copper is used as the anode, and the cathode is made of pure copper. As the current passes through the solution, the impure copper anode dissolves, and the copper ions move towards the cathode and are deposited as pure copper. This process allows for the removal of impurities from the copper, resulting in a higher level of purity than other methods such as roasting or heating with coke.

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The s-block elements are the elements in Group 1 and Group 2 of the periodic table, plus hydrogen and helium. They are called "s-block" elements because their valence (outermost) electrons are in the s subshell of their atoms. Aluminium is not an s-block element. It is located in Group 13, which is part of the p-block elements. Its valence electrons are in the p subshell of its atoms. Therefore, the answer is aluminium.

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The statement that is not true of tin is that it combines with copper to form the alloy brass. Brass is actually an alloy of copper and zinc, not copper and tin. All the other statements are true of tin. Tin is a p-block element, it is extracted from cassiterite SnO₂, it is used for protecting iron containers from corrosion, and it shows two oxidation states, +2 and +4 in most of its compounds.

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The voltage of the cell can be calculated using the formula: E_cell = E_{reduction, cathode} - E_{oxidation, anode} The half-reactions for the cell are: Cathode: Cu²⁺ (aq) + 2e^- → Cu(s) (E_{reduction, cathode} = 0.337V) Anode: Zn(s) → Zn²⁺ (aq) + 2e^- (E_{oxidation, anode} = -0.763V) Substituting the values into the formula, we get: E_cell = 0.337V - (-0.763V) = 1.1V Therefore, the voltage of the cell represented at Zn_(s) Zn ²⁺_(aq) /Cu²⁺_(aq) /Cu_(s) is +1.10V. Answer: +1.10V

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The correct answer is allotropy. Allotropy is a phenomenon in which an element can exist in two or more different physical forms, known as allotropes, in the same physical state (i.e., solid, liquid, or gas) under different conditions of temperature and pressure. The different allotropes of an element have different physical and chemical properties, such as density, melting point, and reactivity, due to differences in their molecular or crystal structures. Some examples of elements that exhibit allotropy include carbon (which has the allotropes graphite, diamond, and fullerenes), oxygen (which has the allotropes O2 and O3), and sulfur (which has the allotropes S8, S6, and S4).

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Among the given options, ethylethanoate, methanol, ethanoic acid, methanoic acid, and ethanol are all polar substances, meaning they have a partially positive and partially negative end due to the arrangement of their atoms. These partial charges allow them to interact with water molecules through hydrogen bonding, making them soluble or miscible in water. However, ethylethanoate is the only exception since it is a nonpolar substance. It lacks partial charges, and as such, cannot form hydrogen bonds with water molecules, making it insoluble or immiscible in water.

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The molar mass of a gas can be calculated using the following formula: molar mass = mass ÷ number of moles At standard temperature and pressure (STP), the molar volume of a gas is 22.4 dm³, which is equivalent to 22,400 cm³. Therefore, the number of moles of the gas can be calculated as follows: number of moles = volume of gas at STP ÷ molar volume of gas at STP number of moles = 200 cm³ ÷ 22,400 cm³/mol number of moles = 0.00893 mol Now, we can use the formula for molar mass to find the answer: molar mass = mass ÷ number of moles molar mass = 0.268 g ÷ 0.00893 mol molar mass = 30.03 g/mol Therefore, the molar mass of the gas is approximately 30 g/mol. The correct answer is.

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There are 6 carbon atoms in a benzene ring. Benzene is a colorless, highly flammable liquid that is composed of carbon and hydrogen atoms. Its molecular formula is C6H6, which means it contains six carbon atoms and six hydrogen atoms. The carbon atoms are arranged in a unique hexagonal ring structure, which is sometimes referred to as an aromatic ring. This ring structure gives benzene its distinctive chemical and physical properties and makes it an important building block for the production of many chemicals and materials, including plastics, rubber, and dyes.

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The formation of allotropes of sulphur is dependent on the concentration of the sulphur solution. If the concentration of the sulphur solution is high, then the allotrope of sulphur formed would be rhombic sulphur. However, if the concentration of the sulphur solution is low, then the allotrope of sulphur formed would be amorphous sulphur. Therefore, the concentration of the solution determines the allotrope of sulphur formed during evaporation.

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Petrol is a complex mixture of hydrocarbons obtained from petroleum, and the composition of petrol can vary depending on its source and refining process. However, hydrocarbons with a higher number of carbon atoms tend to have higher boiling points and are more likely to be present in diesel or other heavier fuels rather than petrol. Therefore, the hydrocarbon that is not likely to be present in petrol is C₁₄H₃₀.

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Octane number is a measure of the performance of petrol in internal combustion engines. It is a rating based on the ability of petrol to resist 'knocking' or 'pinging' during combustion. Knocking or pinging is an undesirable condition that occurs when the air-fuel mixture in the engine's cylinder detonates prematurely, which can cause engine damage. A high octane number indicates that the petrol can resist knocking, allowing the engine to operate at higher compression ratios, which typically results in more power and efficiency. Therefore, petrol containing a high proportion of compounds with high resistance to knocking, such as 2,2,4-trimethylpentane (also known as iso-octane), will have a higher octane number. Hence, the correct option is 2,2,4-trimethylpentane.

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