Stoichiometry
Use balanced equations to relate amounts of reactants and products.
Stoichiometry uses a balanced chemical equation as a recipe. The coefficients give mole ratios: in , two moles of hydrogen react with one mole of oxygen to make two moles of water. Grams do not appear in the ratio, so every calculation goes grams to moles, across the ratio, then back to grams.
When reactants are not in the exact recipe proportions, one runs out first. That limiting reactant sets the maximum amount of product, the theoretical yield. Percent yield compares what was actually collected to that maximum.
Key ideas
- Balance the equation first; never change subscripts to balance.
- Path: grams A moles A moles B grams B.
- Limiting reactant: compute product from each reactant; the smaller amount wins.
- Percent yield .
Worked example
How many grams of water form from 8.0 g of with excess oxygen?
- Moles of : mol.
- Ratio , so mol water.
- Mass: g.
Answer: about g
Common mistakes
- Using grams directly in the mole ratio.
- Working from an unbalanced equation.
- Picking the reactant with the smaller mass as limiting.
Practice questions
Try each one, then open the answer. Numerical answers on this page are verified by an automated calculation check.
1.In , how many moles of come from 6 mol ?
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2.Theoretical yield 50 g, actual 42 g. Percent yield?
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3.In , moles of needed for 5 mol ?
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