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Chemical Calculations

Topic 4 of 5

Use the equation ratio

Convert to moles before comparing reactants.

O-Level 5086 / 5088 (2026) / SEC G3 K326 / K328 (2027)

Use the equation ratio

Convert to moles before comparing reactants.

A reliable calculation path
  1. Write and balance the reaction

    The coefficients give a mole ratio, not a mass ratio.

  2. Convert the known quantity to moles

    Mass: n = m/M. Gas at room temperature and pressure: n = V/24 with V in dm3.

  3. Apply the mole ratio

    For aA -> bB, n(B) = n(A) x b/a.

  4. Convert to the requested quantity

    Use m = nM or V = 24n at room temperature and pressure.

Worked example

Mass to gas volume

Excess hydrochloric acid reacts with 2.40 g Mg. Calculate hydrogen volume at room temperature and pressure; Mg = 24.

  1. Mg(s) + 2HCl(aq) -> MgCl2(aq) + H2(g).
  2. n(Mg) = 2.40/24 = 0.100 mol; the Mg:H2 ratio is 1:1.
  3. V(H2) = 0.100 x 24 = 2.40 dm3.
Answer

2.40 dm3, or 2400 cm3. The 24 dm3 mol-1 value applies at room temperature and pressure.

Worked example

Mass to mass through the mole ratio

What mass of MgO forms when 3.60 g Mg burns completely in excess oxygen? Use Mg = 24 and O = 16.

  1. 2Mg(s) + O2(g) -> 2MgO(s). The Mg:MgO ratio is 2:2, or 1:1.
  2. n(Mg) = 3.60/24 = 0.150 mol, so n(MgO) = 0.150 mol.
  3. M(MgO) = 24 + 16 = 40 g mol-1; mass = 0.150 x 40 = 6.00 g.
Answer

6.00 g MgO. The extra 2.40 g is oxygen taken from the air; the 1:1 mole ratio does not mean equal masses.

Worked example

Identify the limiting reactant

Mix 0.080 mol Mg with 0.100 mol HCl. How much H2 forms?

  1. The equation needs two moles of HCl for each mole of Mg.
  2. 0.080 mol Mg would require 0.160 mol HCl; only 0.100 mol is available. HCl is limiting.
  3. n(H2) = 0.100/2 = 0.050 mol. Mg used = 0.050 mol; Mg remaining = 0.030 mol.
Answer

0.050 mol hydrogen, equivalent to 1.20 dm3 at room temperature and pressure.

Check your understandingCan 0.10 mol of Mg react completely with 0.10 mol of HCl?Think it through, then reveal the answer
No. Mg + 2HCl requires two moles of acid per mole of Mg. The acid consumes only 0.050 mol Mg; HCl is limiting and 0.050 mol Mg remains.