What is stoichiometry?

• A. The calculation of quantities in chemical reactions
• B. The study of reaction rates
• C. The measurement of energy changes in reactions
• D. The arrangement of atoms in molecules

Answer: (A)

The main idea is to calculate quantities in chemical reactions using the balanced equation and mole ratios. When a chemical equation is balanced, the coefficients tell you how many moles of each substance participate. Those numbers become conversion factors that let you relate amounts of one substance to amounts of another, or to grams and even volumes for gases.

For example, in a reaction like 2 H2 + O2 → 2 H2O, the mole ratios are 2:1:2. If you start with 3 moles of hydrogen and 2 moles of oxygen, hydrogen is the limiting reactant because you only need 1.5 moles of O2 to react with 3 moles of H2, leaving 0.5 moles of O2 unused. You’d form 3 moles of water. This is the predictive power of stoichiometry: it tells you how much product can be made from given reactants and how much of each reactant is needed.

Stoichiometry is not about reaction rates (that’s kinetics), nor about energy changes in reactions (thermochemistry), nor about how atoms are arranged in molecules (molecular structure). It specifically uses balanced equations and mole relationships to calculate quantities.