KCSE Chemistry — Practice Paper 1

Short-answer practice across atomic structure, bonding, the mole, acids and bases, redox and organic chemistry — the six areas that carry most of Paper 1.

How to use this: work each one on paper with the working written out. In Chemistry the working is the marks — a correct final answer with no method typically scores 1 of 3.

Section A — Short answer

1. An atom of element X has 17 protons and 20 neutrons. State its mass number and write its electron configuration (2 marks)

• Mass number = protons + neutrons = 17 + 20 = 37
• Electron configuration = 2, 8, 7

Where the marks sit: 1 for the mass number, 1 for the configuration. Note the configuration comes from the proton number, not the mass number — a very common slip under time pressure.

Follow-through: 2,8,7 means 7 outer electrons, so X is a halogen (chlorine) and forms X⁻ by gaining one electron.

2. Explain why sodium chloride conducts electricity when molten or in solution, but not when solid (3 marks)

1. Sodium chloride is ionic, made of Na⁺ and Cl⁻ ions
2. In the solid, the ions are held in fixed positions in a lattice and cannot move
3. When molten or dissolved, the lattice breaks down and the ions are free to move and carry charge

Where the marks sit: 1 for identifying mobile ions as the charge carriers, 1 for the fixed lattice in the solid, 1 for mobility on melting or dissolving.

Common mistake: saying "electrons carry the charge". In ionic conduction the carriers are ions. Electrons carry charge in metals — mixing these up loses all three marks because it is the wrong mechanism, not a wording problem.

3. Calculate the number of moles in 8.0 g of methane, CH₄ (2 marks)

(C = 12, H = 1)

Molar mass of CH₄ = 12 + (4 × 1) = 16 g/mol
moles = mass ÷ molar mass
      = 8.0 ÷ 16
      = 0.5 mol

Where the marks sit: 1 for the molar mass, 1 for the answer. Even if the arithmetic goes wrong, a correctly stated molar mass earns its mark — always write it down.

4. 25.0 cm³ of 0.10 M NaOH is exactly neutralised by 20.0 cm³ of hydrochloric acid. Calculate the concentration of the acid (3 marks)

NaOH + HCl → NaCl + H₂O          (1 : 1)

moles NaOH = (25.0 / 1000) × 0.10 = 2.5 × 10⁻³ mol
∴ moles HCl = 2.5 × 10⁻³ mol      (1 : 1 ratio)

concentration = moles ÷ volume in dm³
              = 2.5 × 10⁻³ ÷ (20.0 / 1000)
              = 0.125 M

Where the marks sit: 1 for moles of NaOH, 1 for using the 1:1 ratio, 1 for the final concentration with units.

Common mistake: forgetting to convert cm³ to dm³ (divide by 1000). This gives an answer 1000× out and costs the accuracy mark, though the method marks survive if the working is visible — another reason to show it.

5. Define oxidation in terms of electrons, and identify the species oxidised in the reaction below (3 marks)

Zn(s) + Cu²⁺(aq) → Zn²⁺(aq) + Cu(s)

• Oxidation is the loss of electrons — 1 mark
• Zn is oxidised: it goes from 0 to +2, losing 2 electrons — 1 mark for the species, 1 mark for the justification

Memory aid: OIL RIG — Oxidation Is Loss, Reduction Is Gain.

Common mistake: naming Cu²⁺ because copper "appears" first in your reading. Track the oxidation number: Cu²⁺ → Cu is +2 → 0, a gain of electrons, so copper is reduced.

6. State two differences between ionic and covalent compounds (2 marks)

Where the marks sit: any 2 contrasted pairs, 1 each — both sides stated.

7. Name the products when ethanol is heated with excess concentrated sulphuric acid at 170 °C, and write the equation (3 marks)

C₂H₅OH  --(conc. H₂SO₄, 170 °C)-->  C₂H₄  +  H₂O
ethanol                              ethene    water

• Product: ethene (and water) — 1 mark
• Correct equation — 1 mark
• Reaction type: dehydration (elimination) — 1 mark

Common mistake: giving ethoxyethane. That is the product at 140 °C; at 170 °C the reaction dehydrates to the alkene. Temperature is part of the question, not decoration.

8. Describe a chemical test for carbon dioxide (2 marks)

• Test: bubble the gas through limewater (calcium hydroxide solution)
• Result: the limewater turns milky / white precipitate forms

Where the marks sit: 1 for the reagent, 1 for the observation. An observation without the reagent scores nothing — the marker cannot tell what you did.

9. Explain why Group I metals become more reactive down the group (3 marks)

1. Going down the group, atoms have more electron shells, so the outer electron is further from the nucleus
2. There is greater shielding by inner shells
3. The outer electron is therefore less strongly attracted and is lost more easily, so reactivity increases

Where the marks sit: 1 for atomic radius, 1 for shielding, 1 for ease of electron loss. The third mark requires you to connect the first two to reactivity — many answers describe the structure and stop.

10. A hydrocarbon contains 85.7% carbon and 14.3% hydrogen by mass. Determine its empirical formula (3 marks)

(C = 12, H = 1)

Empirical formula = CH₂

Where the marks sit: 1 for dividing by relative atomic masses, 1 for dividing by the smallest, 1 for the formula.

Note: CH₂ cannot exist alone — the molecular formula is a multiple of it (C₂H₄, C₃H₆ …). If a question gives you the relative molecular mass, divide it by 14 to find which.

What separates a B from an A in Paper 1

1. Show every line of working. Method marks are awarded independently of the final answer. A slip in arithmetic costs one mark if the method is visible, and all of them if it is not.
2. Carry units through. An answer without units is incomplete in a quantitative question.
3. Read the conditions. Temperature, concentration and "excess" are given because they change the product.

Where to go next

• KCSE Physics Practice Paper 1 — the same discipline about units and working
• KCSE Biology Practice Paper 1 — short-answer wording practice

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