Stoichiometry: balancing and calculating

Stoichiometry is the arithmetic of reactions: using a balanced equation to work out how much reactant you need or product you'll get. The method is always the same — balance the equation, convert to moles, apply the ratio, convert back.

Balancing equations

A balanced equation has equal numbers of each atom on both sides, because mass is conserved. Adjust the big numbers in front (the coefficients) — never the small subscripts inside a formula, which would change the substance. Check each element in turn, leaving oxygen and hydrogen until last.

The mole-ratio method

Once balanced, the equation gives a ratio in moles. The standard calculation: convert the given quantity to moles, multiply by the ratio from the equation, then convert that result back to mass, gas volume (÷24 dm³ at rtp) or concentration. Almost every calculation question is a version of these three steps.

Limiting reagent and yield

When two reactant amounts are given, one usually runs out first — the limiting reagent — and it controls how much product forms. Work out the moles of each, compare against the equation's ratio, and base the answer on the limiting one. Percentage yield compares the product you actually got with the maximum the equation predicts.

Common slips

Marks go on using an unbalanced equation, forgetting to identify the limiting reagent, and the usual unit errors (cm³ not converted to dm³, dropped units, wrong significant figures). Lay the working out clearly with units at each line — examiners award method marks even when the final number slips.