Stoichiometry

MDCAT Chemistry · Chemistry · Fundamental concepts of chemistry. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

Test this topic free All MDCAT Chemistry topics

The idea

A balanced equation is a recipe written in moles. The numbers in front of the formulae give the mole ratio in which the substances react and form. Masses do not react in simple ratios, so every calculation goes through moles.

When the reactants are not mixed in the exact ratio, one of them runs out first. This is the limiting reagent. It decides how much product can form, and some of the other reactant is left over.

The amount of product worked out from the equation is the theoretical yield. The amount really collected, the actual yield, is usually smaller because of side reactions, reversible reactions and losses in handling.

Rules to remember

Coefficients in a balanced equation = ratio of moles
Route: mass → moles → mole ratio → moles → mass or gas volume
For gases at the same temperature and pressure, the volume ratio = the mole ratio
Limiting reagent: divide the moles of each reactant by its coefficient; the smallest answer shows the limiting reagent
Theoretical yield is calculated from the limiting reagent only
percentage yield = (actual yield ÷ theoretical yield) × 100
Mass is conserved: total mass of reactants used = total mass of products formed

Common mistake

Students pick the limiting reagent by comparing masses. Change both masses into moles first, then compare them with the ratio in the equation.

Worked example

4.8 g of magnesium is heated with 1.6 g of oxygen: 2Mg + O₂ → 2MgO. What is the maximum mass of magnesium oxide that can form? (O = 16, Mg = 24)

Moles: Mg = 4.8 ÷ 24 = 0.20 mol; O₂ = 1.6 ÷ 32 = 0.050 mol.
0.050 mol of O₂ needs only 2 × 0.050 = 0.10 mol of Mg, so oxygen is the limiting reagent and magnesium is in excess.
Moles of MgO = 2 × 0.050 = 0.10 mol.
Mass of MgO = 0.10 × (24 + 16) = 0.10 × 40 = 4.0 g.

Answer: 4.0 g of MgO (2.4 g of magnesium is left over)

Ask about this

Type what you are stuck on. The helper finds the explanation, worked examples and the notes to read, free and instant.

Keep going

← The mole Particles, photons and the Bohr model →Take a MDCAT Chemistry test