Solutions and gas volumes — GCSE Chemistry practice questions

Question 1

A sample of sea water contains 36.0 g of dissolved salts in every 1 dm³. A student evaporates 250 cm³ of this sea water to dryness.

What mass of solid salts is left?

Accepted Answer

B: 9.00 g. 250 cm³ = 0.250 dm³. Each dm³ holds 36.0 g of salts, so 0.250 dm³ holds 36.0 × 0.250 = 9.00 g.

Question 2

An oxygen candle on a space station releases oxygen when sodium chlorate, NaClO₃, is heated.

2NaClO₃ → 2NaCl + 3O₂

A small test candle contains 2.13 g of sodium chlorate. [Mr: NaClO₃, 106.5]

One mole of any gas occupies 24 dm³ at r.t.p.

What volume of oxygen, in cm³ at r.t.p., is released when all the sodium chlorate decomposes?

Accepted Answer

D: 720 cm³. Moles of NaClO₃ = 2.13 ÷ 106.5 = 0.0200 mol, so moles of O₂ = 0.0200 × 3 ÷ 2 = 0.0300 mol. Volume = 0.0300 × 24 000 = 720 cm³.

Question 3

A kettle descaler contains ethanoic acid. It removes limescale (calcium carbonate).

CaCO₃ + 2CH₃COOH → Ca(CH₃COO)₂ + H₂O + CO₂

A 1.50 g piece of limescale is added to 20.0 cm³ of 1.00 mol/dm³ ethanoic acid.

One mole of any gas occupies 24 dm³ at r.t.p.

What is the maximum volume of carbon dioxide, in cm³ at r.t.p., that can be made?

Accepted Answer

B: 240 cm³. Moles of CaCO₃ = 1.50 ÷ 100 = 0.0150 mol; moles of acid = 1.00 × 20.0 ÷ 1000 = 0.0200 mol. All the limescale would need 0.0300 mol of acid, so the acid is limiting and gives 0.0200 ÷ 2 = 0.0100 mol of CO₂ = 0.0100 × 24 000 = 240 cm³.