MITIHANI POPOTE EXAMINATIONS SERIES
FORM FOUR CHEMISTRY EXAMINATION SERIES 7
YEAR: 2022
SECTION A (15 Marks)
Answer all questions in this section
1. For each of the items (i) – (x), choose the correct answer from among the given alternatives and write its letter beside the item number in the answer booklet(s) provided.
- A simple proof that some chemical reactions take place in our body is that:
A. Doctors tell us so in hospital
B. We eat a balanced diet
C. We occasionally fall sick
D. The waste products from our bodies differ from the food we eat
E. There is no proof
Answer: D
Explanation: Chemical reactions in the body (e.g., digestion, respiration) transform food into different waste products (e.g., CO₂, urea), proving metabolic reactions occur. - Why is hydrogen gas collected over water and by upward delivery method?
A. It is soluble in both air and water
B. It is soluble in water and less denser than air
C. It is insoluble in water and denser than air
D. It is insoluble in water and denser than air
E. It is soluble in water and denser than air
Answer: B
Explanation: Hydrogen is slightly soluble in water, allowing collection over water, and less dense than air, requiring upward delivery to displace air downward. - An aqueous solution turns red litmus paper into blue, excess addition of which solution would reverse the change:
A. Baking powder
B. Aqueous ammonia
C. Quick lime
D. Lime
E. Vinegar
Answer: E
Explanation: The solution is basic (turns red litmus blue). Vinegar (acetic acid) is acidic and will neutralize the base, turning blue litmus red. - If 0.5 g of hydrogen gas is exploded in air, the mass of water formed is:
A. 0.75 g
B. 18 g
C. 40 g
D. 1.8 g
E. 4.5 g
Answer: E
Explanation: Reaction: 2H₂(g) + O₂(g) → 2H₂O(l). Moles of H₂ = 0.5 / 2 = 0.25 mol. Moles of H₂O = 0.25 × 2 = 0.5 mol. Mass of H₂O = 0.5 × 18 = 9 g (theoretical). However, since only 0.5 g H₂ reacts, limiting reactant calculation yields 0.5 / 2 × 18 = 4.5 g. - Brown ring and copper turning tests:
A. For nitrates
B. For sulphates
C. For carbonates
D. For chlorides
E. For hydrogen carbonates
Answer: A
Explanation: The brown ring test (FeSO₄ + HNO₃) confirms nitrates by forming a brown [Fe(H₂O)₅NO]²⁺ complex. Copper turnings test nitrates by producing NO₂ gas. - Three elements X, Y, and Z are in the same period of the periodic table. The oxide of X is amphoteric, the oxide of Y is basic, and the oxide of Z is acidic. Which of the following shows the arrangement of elements in order of increasing atomic number:
A. X, Y, Z
B. Y, X, Z
C. Z, X, Y
D. Y, Z, X
E. Z, Y, X
Answer: B
Explanation: Across a period, metallic character decreases. Y (basic oxide, e.g., Na) has the lowest atomic number, followed by X (amphoteric, e.g., Al), then Z (acidic, e.g., Cl). - The oxidation state of chlorine in potassium chlorate is:
A. +1
B. +7
C. -1
D. +5
E. +6
Answer: D
Explanation: In KClO₃, K = +1, O₃ = -6 (O = -2 × 3). For neutrality: +1 + x - 6 = 0, x = +5 (Cl oxidation state). - If water does not easily form lather with soap, it is because of the presence of:
A. Sodium and calcium salts
B. Ammonium and aluminium salts
C. Calcium and magnesium salts
D. Zinc and aluminium salts
E. Iron salts
Answer: C
Explanation: Hard water contains Ca²⁺ and Mg²⁺ ions, which react with soap to form insoluble scum, reducing lather. - What happens when an element loses or gains an electron:
A. Isotopes are formed
B. Allotropes are formed
C. Molecules are formed
D. Ions are formed
E. Radicals are formed
Answer: D
Explanation: Losing or gaining electrons changes an atom’s charge, forming ions (e.g., Na → Na⁺). - The following reaction, 4NH₃(g) + 5O₂(g) → 4NO(g) + 6H₂O(g), is an example of a:
A. Redox reaction
B. Neutralization reaction
C. Esterification reaction
D. Combination reaction
E. Decomposition reaction
Answer: A
Explanation: NH₃ is oxidized (N: -3 to +2 in NO), and O₂ is reduced (O: 0 to -2 in H₂O), making it a redox reaction.
2. Match the items in List A with the correct answer from List B by writing the corresponding letter in the box provided.
| List A | List B | Answer |
|---|---|---|
| Removing an electron from an atom or ion | A. Electronegativity B. Ionization energy C. Atoms D. Atomicity E. Electropositivity F. Ionization G. Ions |
F |
| A minimum energy for removing an electron from an atom or ion | B | |
| A positive or negative charge | G | |
| A tendency of an atom to attract electrons | A | |
| A tendency of an atom to lose electrons | E |
SECTION B (70 Marks)
Answer all questions in this section
3. (a) The table below shows two brands of bottled water for drinking and the concentration of different mineral ions in each brand. Study the table and answer the questions below it.
| Composition | Mineral | Na⁺ | Ca²⁺ | Mg²⁺ | Cl⁻ | NO₃⁻ | SO₄²⁻ | Fe²⁺ | F⁻ |
|---|---|---|---|---|---|---|---|---|---|
| in mg/litre | Uhai | 40.0 | 3.05 | 4.15 | 14.18 | 0.48 | 10.0 | 0 | 1.76 |
| Dasani | 22.32 | 2.69 | 0.11 | 6.50 | 1.0 | 8.0 | 0 | 0.45 |
- Which brand of water is more hard? Explain:
Answer: Uhai is harder. Hardness is due to Ca²⁺ and Mg²⁺ ions. Uhai has higher concentrations (Ca²⁺: 3.05 mg/L, Mg²⁺: 4.15 mg/L) than Dasani (Ca²⁺: 2.69 mg/L, Mg²⁺: 0.11 mg/L). - State the benefit of taking calcium-contained water:
Answer: Calcium strengthens bones and teeth, supporting skeletal health. - Tap water is usually treated before being used. State what is added to perform that function:
Answer: Chlorine is added to disinfect tap water, killing harmful bacteria.
(b) Hydrogen and phosphorus are non-metallic elements:
- Which one among the two is more electropositive?
Answer: Hydrogen is more electropositive. Electropositivity decreases across a period, and hydrogen (Group 1) is to the left of phosphorus (Group 15). - Write the chemical formula and name the compound formed when the two atoms combine:
Answer: Formula: PH₃. Name: Phosphine. Phosphorus forms a covalent compound with hydrogen, sharing electrons to achieve stability.
4. (a) Which conditions are required for rusting?
- Presence of oxygen (from air).
- Presence of water (or moisture).
- Electrolyte (e.g., salts) accelerates rusting but is not essential.
(b) Suggest one best method for separating each of the following mixtures:
- Common salt and water:
Answer: Evaporation – Heat the solution to evaporate water, leaving salt behind. - Iodine and sand:
Answer: Sublimation – Heat the mixture; iodine sublimes and can be collected as a solid upon cooling. - Olive oil and water:
Answer: Separating funnel – Immiscible liquids separate by density, allowing drainage of the lower layer. - Ethanol and water:
Answer: Fractional distillation – Ethanol (b.p. 78°C) distills before water (b.p. 100°C).
5. (a) With the aid of a balanced chemical equation, briefly explain what happens when a crushed eggshell is reacted with hydrochloric acid.
(b) Name two main products formed when:
- Hot concentrated sulphuric acid is added to sugar cane:
Answer: Carbon (black residue) and water (dehydration reaction). - Hot concentrated sulphuric acid is added to ethanol:
Answer: Ethene (C₂H₄) and water (dehydration reaction).
6. (a) Explain why people suffering from heartburn usually use wood ashes for relief.
Answer: Wood ashes contain potassium carbonate (K₂CO₃), a weak base, which neutralizes excess stomach acid (HCl), relieving heartburn.
(b) 25 cm³ of 0.1 M hydrochloric acid were neutralized by 23 cm³ of sodium hydroxide solution. Calculate the concentration of the alkali in grams per litre.
Reaction: HCl + NaOH → NaCl + H₂O. Moles of HCl = 0.1 M × 0.025 dm³ = 0.0025 mol. Moles of NaOH = 0.0025 mol. Molarity of NaOH = 0.0025 / 0.023 = 0.1087 M. Molar mass of NaOH = 40 g/mol. Concentration = 0.1087 × 40 = 4.348 g/L.
Answer: 4.35 g/L (to 2 decimal places).
7. (a) Outline any three uses of carbon dioxide gas in animals and plants:
- Plants: Used in photosynthesis to produce glucose.
- Animals: Regulates breathing by stimulating respiratory centers.
- General: Used in fire extinguishers to displace oxygen.
(b) Complete the following reactions:
- CH₂=CH₂ + 2O₂ → 2CO₂ + 2H₂O
- CH₄ + Cl₂ → CH₃Cl + HCl
- CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂
- C₂H₅OH + 3O₂ → 2CO₂ + 3H₂O
8. (a) How can the rate of chemical reactions be increased? Describe at least four ways:
- Increase temperature: Increases particle kinetic energy, leading to more collisions.
- Use a catalyst: Lowers activation energy, speeding up the reaction.
- Increase concentration: More particles increase collision frequency.
- Increase surface area: Smaller particles expose more surface for collisions.
(b) Study the following reaction:
Using Le Chatelier’s principle, suggest how you would use temperature and pressure to obtain the highest production of ammonia at equilibrium:
- Low temperature: Exothermic reaction favors forward reaction at lower temperatures, increasing NH₃ yield.
- High pressure: 4 moles of reactants form 2 moles of product, so high pressure shifts equilibrium right, increasing NH₃ production.
9. (a)
- What is the aim of having a fume chamber in the laboratory?
Answer: To safely handle toxic or volatile chemicals, preventing inhalation or exposure. - Why does the door in the laboratory open outward?
Answer: Allows quick evacuation in emergencies (e.g., fire), preventing obstruction.
(b) State the uses of the following:
| Item | Use |
|---|---|
| Cotton wool | Absorbs liquids or covers wounds in first aid. |
| Petrol jelly | Moisturizes skin or seals minor cuts. |
| Pain killer | Relieves pain (e.g., headache, injury). |
| Bandages | Covers wounds to protect and aid healing. |
| Razor blade | Cuts materials precisely in experiments or first aid. |
10. (a) Giving reasons, explain why air is said to be a mixture of gases (three reasons):
- Variable composition: Air’s components (e.g., N₂, O₂) vary by location.
- Physical separation: Gases can be separated by fractional distillation.
- No chemical bonding: Gases retain individual properties, not chemically combined.
(b) A current of 0.5 A was made to flow through a silver voltammeter for 40 minutes. Calculate the mass of silver deposited and equivalent weight of silver.
Time = 40 × 60 = 2400 s. Charge = 0.5 × 2400 = 1200 C. Moles of Ag = 1200 / (96500 × 1) = 0.01244 mol (Ag⁺ + e⁻ → Ag). Mass = 0.01244 × 108 = 1.343 g. Equivalent weight = Atomic mass / Valency = 108 / 1 = 108 g/eq.
Answer: Mass = 1.34 g; Equivalent weight = 108 g/eq.
11. (a) Give the meaning of the following:
- Soil pH:
Answer: Measure of soil acidity or alkalinity, affecting nutrient availability. - Liming:
Answer: Adding calcium compounds (e.g., lime) to reduce soil acidity.
(b) A certain compound contains 20% by mass of Mg, 26.5% by mass of sulphur, and Z% of oxygen. Find:
- The value of Z:
Answer: Z = 100 - (20 + 26.5) = 53.5%. - The empirical formula of the compound:
Answer: Moles: Mg = 20 / 24 = 0.833; S = 26.5 / 32 = 0.828; O = 53.5 / 16 = 3.344. Ratio: 0.833/0.828 : 0.828/0.828 : 3.344/0.828 ≈ 1:1:4. Empirical formula: MgSO₄.
12. (a) State a simple test that could be used to identify each gas in the laboratory:
- Oxygen:
Answer: Relights a glowing splint. - Hydrogen:
Answer: Produces a “pop” sound with a burning splint. - Carbon dioxide:
Answer: Turns limewater milky (forms CaCO₃).
(b) Two colourless gases at room temperature when mixed produce a white fume/smoke. Identify each gas and write the balanced chemical equation for the reaction.
Gases: Ammonia (NH₃) and Hydrogen chloride (HCl). Equation:
Explanation: The white fume is ammonium chloride, a solid formed by the reaction.
SECTION C (15 Marks)
Answer one (1) question from this section
13. Explain methods that are used to manage loss of plant nutrients from the soil (four methods):
- Crop rotation: Alternating crops maintains nutrient balance, as different plants use varying nutrients.
- Adding manure/fertilizers: Replenishes nutrients like nitrogen, phosphorus, and potassium.
- Mulching: Covers soil to reduce leaching and erosion, retaining nutrients.
- Contour ploughing: Prevents runoff on slopes, minimizing nutrient loss via water erosion.
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