Stoichiometry (Full)

Stoichiometry is the part of chemistry that uses a balanced chemical equation to connect amounts of substances in a reaction. It lets you predict how much reactant is needed, how much product can form, and which substance runs out first. These ideas matter in labs, industry, medicine, and environmental science because chemical reactions must be measured accurately. A balanced equation acts like a map that links particles, moles, mass, and volume.

The key tool in stoichiometry is the mole ratio, which comes from the coefficients in a balanced equation. You often convert from grams to moles, use the mole ratio to switch substances, and then convert to the desired unit such as grams or liters. When reactants are mixed, the limiting reactant is the one consumed first and it determines the maximum possible product, called the theoretical yield. Real experiments usually produce less than this amount, so percent yield compares actual yield to theoretical yield.

Key Facts

Coefficients in a balanced equation give mole ratios between substances.
Moles = mass / molar mass
mass = moles x molar mass
For aA + bB -> cC + dD, mole ratio A:B:C:D = a:b:c:d
Percent yield = (actual yield / theoretical yield) x 100%
The limiting reactant is the reactant that produces the smaller amount of product when each reactant is calculated separately.

Vocabulary

Stoichiometry: Stoichiometry is the calculation of amounts of reactants and products using a balanced chemical equation.
Mole ratio: A mole ratio is the ratio between coefficients in a balanced equation and is used to convert between substances.
Limiting reactant: The limiting reactant is the reactant that is used up first and stops the reaction from making more product.
Theoretical yield: Theoretical yield is the maximum amount of product predicted by stoichiometric calculation.
Percent yield: Percent yield is the percentage of the theoretical yield that is actually obtained in an experiment.

Common Mistakes to Avoid

Using an unbalanced equation, which gives wrong mole ratios and makes every later calculation incorrect. Always balance the reaction before doing any stoichiometry.
Skipping the conversion to moles, which is wrong because mole ratios only apply to moles, not directly to grams or liters. Convert given amounts to moles first unless the problem already gives moles.
Choosing the limiting reactant by comparing masses alone, which is wrong because different substances have different molar masses and coefficients. Calculate how much product each reactant can make before deciding.
Using actual yield in place of theoretical yield, which is wrong because percent yield needs both values in the correct positions. The formula is percent yield = actual/theoretical x 100%.

Practice Questions

1 For the reaction 2H2 + O2 -> 2H2O, how many moles of H2O can form from 5.0 mol H2 if O2 is in excess?
2 For the reaction N2 + 3H2 -> 2NH3, if 14.0 g N2 reacts with excess H2, what mass of NH3 can be produced?
3 A reaction mixture contains two reactants, and one is present in a larger mass than the other. Explain why the reactant with the larger mass is not always the excess reactant.