The coefficients of a balanced equation give the mole ratios of reactants and products. These ratios give how much product forms, and show which reactant is used up first and limits the yield.
Methane burns in oxygen:
CH₄ + 2O₂ → CO₂ + 2H₂O
The numbers in front of the formulae are the stoichiometric coefficients. They give the ratio of particles, and therefore the ratio of moles. The equation states: 1 mol of methane reacts with 2 mol of oxygen to give 1 mol of carbon dioxide and 2 mol of water.
Stoichiometry is the calculation of the amounts of reactants and products in a reaction, using these ratios.
A recipe is a useful comparison. 2 eggs and 1 cup of flour make one cake; doubling one ingredient means doubling the others. The comparison holds for ratios. It breaks down in two places: an equation’s ratios are in moles, not in grams or cups, and in a reaction every atom of the reactants appears in the products.
Example: how much water forms when 1 mol of methane burns completely? The ratio CH₄ : H₂O is 1 : 2, so 2 mol of water forms. The molar mass of water is 18 g/mol, so that is 36 g.
From CH₄ + 2O₂ → CO₂ + 2H₂O, how many moles of oxygen are needed to burn 3 mol of methane completely?
The ratio CH₄ : O₂ is 1 : 2.
3 mol of methane needs 3 × 2 = 6 mol of oxygen.
That is 6 × 32 g/mol = 192 g of oxygen. The mole ratio 1 : 2 stays fixed; the amounts scale together.
Reactants are often not mixed in exactly the ratio of the equation. Then one reactant is used up before the other. The reaction stops when that happens, however much of the other reactant remains.
The reactant that is used up first is the limiting reagent. It sets the amount of product. The other reactant is in excess; some of it is left unreacted.
To find the limiting reagent, calculate how much of the second reactant the first one needs. Compare that with how much of the second reactant there is.
In 2H₂ + O₂ → 2H₂O, 4 mol of H₂ is mixed with 4 mol of O₂. Which is the limiting reagent?
The ratio H₂ : O₂ is 2 : 1. So 4 mol of H₂ needs 4 ÷ 2 = 2 mol of O₂.
There is 4 mol of O₂, more than the 2 mol needed. So hydrogen is used up first and is the limiting reagent.
4 mol of H₂ gives 4 mol of water. 4 − 2 = 2 mol of O₂ is left over.
Equal amounts of the two reactants do not mean they react completely. The mole ratio in the equation decides which runs out.
Many reactions are carried out in solution. Then the amount of a reactant depends on how much of it is dissolved in a given volume. This is its concentration.
Molarity is used most in stoichiometry, because moles = molarity × volume in litres. 250 mL of 0.2 M NaOH contains 0.2 × 0.250 = 0.05 mol of NaOH.
Matter is classified into mixtures and pure substances. Measured values carry units and a stated precision. The laws of chemical combination led to the atomic model. The mole links the mass of a sample to its number of particles. Percentage composition gives formulae, and stoichiometry gives the amounts that react and form.
Almost every calculation in the unit passes through the mole, because it connects the mass that can be weighed to the number of particles that react.
1The coefficients in a balanced equation give…
2From CH₄ + 2O₂ → CO₂ + 2H₂O, how many moles of oxygen are needed to burn 3 mol of methane?
3The reactant that is used up first and sets the amount of product is the ____ reagent.
4In 2H₂ + O₂ → 2H₂O, 4 mol H₂ is mixed with 4 mol O₂. The limiting reagent is…
5Molarity is…
6Why is the mole central to stoichiometry?
That completes the unit on the basic concepts of chemistry.
Noescio