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Mass ratios in reactions explained

Syllabus VMBO: Basic chemistry · production processes

Calculate directly from a supplied mass ratio without first converting to mol.

Read what the ratio means

In this numerical model magnesium reacts with oxygen in the rounded mass ratio 3 : 2. Every 3 g of magnesium therefore needs 2 g of oxygen. This ratio is supplied; it is not the ratio of equation coefficients.

Count only reacting mass in the product

For complete reaction without loss, product mass equals the sum of the reactant masses consumed. The leftover is still a reactant. Total mass, including excess, is conserved.

Worked example

12.0 g magnesium and 10.0 g oxygen; mass ratio 3 : 2. Magnesium oxide is the only product.

  1. Oxygen needed: 12.0 × 2/3 = 8.0 g.
  2. Remaining: 10.0 − 8.0 = 2.0 g oxygen.

20.0 g magnesium oxide and 2.0 g oxygen: 22.0 g in total.