Stoichiometry

Stoichiometry rests on conservation of mass: atoms are rearranged in a reaction, never created or destroyed, so a balanced equation must have the same count of each element on both sides. The coefficients in that balanced equation state the ratio in which particles react. For 2 H₂ + O₂ → 2 H₂O, the coefficients say two moles of hydrogen react with one mole of oxygen to give two moles of water — a 2 : 1 : 2 mole ratio.

Because chemists cannot count molecules directly, the mole bridges the gap between the microscopic ratio and measurable mass. One mole is 6.022 × 10²³ particles (Avogadro's number), and the molar mass in grams per mole converts between mass and moles. Nearly every stoichiometry problem follows the same three steps: convert the given quantity to moles, apply the mole ratio from the balanced equation, then convert the resulting moles to whatever unit the question asks for. For gases at a stated temperature and pressure, the ideal gas law supplies the mass-to-volume link; for solutions, molarity does.

Two refinements handle real reactions. The limiting reagent is the reactant that runs out first and therefore caps how much product can form; identifying it means comparing how many moles of product each reactant could produce and taking the smaller value. The leftover reactant is in excess. Percent yield compares what was actually obtained to the theoretical maximum: percent yield = (actual yield ÷ theoretical yield) × 100. Related calculations include determining empirical formulas from percent composition and scaling an empirical formula up to a molecular formula using the measured molar mass.

The MCAT tests stoichiometry in the chemical and physical foundations section, usually embedded in a passage rather than as a standalone calculation. Because no calculator is permitted, the exam rewards estimation and unit reasoning — recognizing which reactant is limiting, tracking mole ratios, and approximating molar masses — over long arithmetic.