Shows that one mole of magnesium reacts with two moles of hydrochloric…

The idea

The coefficients in a balanced equation give the mole ratio for the reaction. In Mg + 2HCl → MgCl₂ + H₂, one mole of magnesium atoms reacts with two moles of hydrochloric acid molecules. This produces one mole of magnesium chloride formula units and one mole of hydrogen molecules. The ratio is 1:2:1:1. Use it by converting any given mass to moles, multiplying or dividing by the ratio, then converting to the requested mass. For example, 0.25 mol Mg needs 0.50 mol HCl and gives 0.25 mol H₂, which has a mass of 0.50 g. This fundamental principle allows chemists to predict the exact mass of product formed or reactant required, ensuring no materials are wasted.

What earns the marks

• Identify the coefficients 1, 2, 1 and 1 as the mole ratio for Mg, HCl, MgCl₂ and H₂ respectively.
• Convert a stated mass of magnesium to moles using moles = mass ÷ relative atomic mass, then double that value to find moles of HCl.
• Use the same ratio to find the moles of MgCl₂ and H₂ produced, then convert to mass as required.
• Explain that the ratio comes from the balanced equation and applies to particles or moles, not directly to masses of different substances.

Common mistakes and how to put them right

• Treating the coefficient 2 in 2HCl as applying to both H and Cl separately.

  Put it right by remembering it multiplies the whole HCl formula unit.

• Using a 1:1 ratio between magnesium and hydrochloric acid.

  Put it right by reading the coefficients and using 1 mol Mg to 2 mol HCl.

• Adding masses directly from the ratio, such as assuming 1 g Mg gives 2 g HCl.

  Put it right by converting to moles before applying the ratio.

Exam tips

• Write the mole ratio underneath the equation before substituting any numbers.
• Keep track of which substance is given and which is asked for, then apply the ratio in the correct direction.
• Always convert mass to moles first, apply the ratio, and then convert back to mass.