Stoichiometry: masses from equations
The coefficients of an equation are for moles, not masses. So always go via moles: grams → mol → ratio → mol → grams.
Formulas and rules
- The equation must be balanced
- n(given) = m / M
- n(wanted) = n(given) × ν(wanted) / ν(given)
- m(wanted) = n(wanted) × M(wanted)
Your turn
Three problems on this topic. Type numbers like 12.5 or 1,250 and elasticities with their sign.
Task
Burning hydrogen: 2 H₂ + O₂ → 2 H₂O. How much H₂O can at most be formed from 192 g of H₂? Atomic masses (g/mol): H 1.01 · O 16.00.
Amount of H₂ in mol (4 decimal places)
Mass of H₂O in g (2 decimal places)
Numbers like 12.5 or 0.025, with as many decimals as asked. Masses and amounts count within 1%, pH values within ±0.02.
Worked example
Neutralisation: NaOH + HCl → NaCl + H₂O. How much NaCl can at most be formed from 118.1 g of HCl? Atomic masses (g/mol): H 1.01 · Na 22.99 · Cl 35.45.
Worked solution
- 1.M(HCl) = 1.01 + 35.45 = 36.46 g/mol
- 2.M(NaCl) = 22.99 + 35.45 = 58.44 g/mol
- 3.n(HCl) = 118.1 / 36.46 = 3.23917 mol
- 4.Ratio from the equation: n(NaCl) = n(HCl) × 1/1 = 3.23917 mol
- 5.m(NaCl) = 3.23917 × 58.44 = 189.3 g
Typical mistake: Using masses instead of amounts in the ratio. The coefficients of the equation are for moles, never for grams.
Common questions
Why can I not work in grams directly?
Particles weigh different amounts. 2 H₂ + O₂ → 2 H₂O means 2 mol hydrogen to 1 mol oxygen, but 4 g to 32 g.