Paper 2 Stoichiometry MCQ Practice

Ten multiple-choice questions in the style of IGCSE Chemistry Paper 2. Choose the best answer for each, then check the mark scheme.

Question 1

What is the relative molecular mass (Mr) of calcium carbonate, CaCO₃? (Ar: Ca = 40, C = 12, O = 16)

A. 68 B. 84 C. 100 D. 116

Answer

C

Mr of CaCO₃ = 40 + 12 + (3 x 16) = 40 + 12 + 48 = 100. Add the relative atomic masses of all atoms in the formula.

Question 2

How many moles of water molecules are in 36 g of water? (Mr of H₂O = 18)

A. 0.5 B. 1 C. 2 D. 18

Answer

C

Moles = mass / Mr = 36 / 18 = 2 mol. One mole of water has a mass of 18 g, so 36 g is two moles.

Question 3

Which equation is correctly balanced?

A. Na + H₂O → NaOH + H₂ B. 2Na + 2H₂O → 2NaOH + H₂ C. Na + 2H₂O → NaOH + H₂ D. 2Na + H₂O → 2NaOH + H₂

Answer

B

Checking option B: Left side has 2 Na, 4 H, 2 O. Right side has 2 Na, 2 O, 2 H (in NaOH) + 2 H (in H₂) = 4 H, 2 O. All atoms balance. The other options have unequal numbers of atoms on each side.

Question 4

What mass of magnesium oxide is formed when 24 g of magnesium reacts completely with oxygen? (Ar: Mg = 24, O = 16; equation: 2Mg + O₂ → 2MgO)

A. 16 g B. 32 g C. 40 g D. 80 g

Answer

C

Moles of Mg = 24 / 24 = 1 mol. From the equation, 2 mol Mg produces 2 mol MgO, so 1 mol Mg produces 1 mol MgO. Mass of MgO = 1 x (24 + 16) = 40 g.

Question 5

What is the empirical formula of a compound containing 40% carbon, 6.7% hydrogen, and 53.3% oxygen by mass? (Ar: C = 12, H = 1, O = 16)

A. CHO B. CH₂O C. C₂H₄O₂ D. CH₃O

Answer

B

C: 40/12 = 3.33; H: 6.7/1 = 6.7; O: 53.3/16 = 3.33. Divide by smallest (3.33): C = 1, H = 2, O = 1. Empirical formula = CH₂O. Option C (C₂H₄O₂) is the molecular formula of ethanoic acid, not the simplest ratio.

Question 6

One mole of any substance contains the same number of particles. This number is:

A. 6.02 x 10²⁰ B. 6.02 x 10²³ C. 6.02 x 10²⁶ D. 6.02 x 10¹²

Answer

B

Avogadro’s constant is 6.02 x 10²³ mol⁻¹. One mole of any substance contains 6.02 x 10²³ particles (atoms, molecules, ions, or formula units, depending on the substance).

Question 7

A solution has a concentration of 2 mol/dm³. What mass of sodium hydroxide (NaOH) is dissolved in 500 cm³ of this solution? (Mr of NaOH = 40)

A. 20 g B. 40 g C. 80 g D. 160 g

Answer

B

Moles = concentration x volume = 2 x 0.5 = 1 mol (convert 500 cm³ to 0.5 dm³). Mass = moles x Mr = 1 x 40 = 40 g.

Question 8

In the reaction CaCO₃ → CaO + CO₂, the theoretical yield of CaO from 100 g of CaCO₃ is 56 g. A student obtains 42 g. What is the percentage yield?

A. 42% B. 56% C. 75% D. 133%

Answer

C

Percentage yield = (actual yield / theoretical yield) x 100 = (42 / 56) x 100 = 75%. Percentage yield is always less than or equal to 100%. A value above 100% would indicate an error or impurity.

Question 9

What is the mass of one mole of sulfuric acid, H₂SO₄? (Ar: H = 1, S = 32, O = 16)

A. 49 g B. 82 g C. 96 g D. 98 g

Answer

D

Mr of H₂SO₄ = (2 x 1) + 32 + (4 x 16) = 2 + 32 + 64 = 98. One mole of H₂SO₄ has a mass of 98 g.

Question 10

In the equation N₂ + 3H₂ → 2NH₃, what is the molar ratio of nitrogen to ammonia?

A. 1 : 1 B. 1 : 2 C. 1 : 3 D. 3 : 2

Answer

B

The coefficients in the balanced equation give the molar ratio. N₂ has a coefficient of 1 and NH₃ has a coefficient of 2, so the ratio of nitrogen to ammonia is 1 : 2. One mole of nitrogen produces two moles of ammonia.