ACT Science: Stoichiometry—Using Mole Ratios to Convert Between Reactants and Products

A balanced chemical equation shows the ratio of moles of reactants to products. Example: 2H₂+O₂→2H₂O means 2 moles of H₂ react with 1 mole of O₂ to produce 2 moles of H₂O. The mole ratio is 2:1:2 (H₂:O₂:H₂O). To solve stoichiometry problems: (1) Write the balanced equation. (2) Identify mole ratio from coefficients. (3) Use ratio to convert between substances. Example: If you have 5 moles of H₂, how many moles of H₂O are produced? Ratio is 2:2 (H₂:H₂O) or 1:1. So 5 moles H₂ produces 5 moles H₂O.

Mole ratios are the bridge between quantities of different substances in a reaction.

Scenario 1 (Reactant to product): 2Na+2HCl→2NaCl+H₂. If you have 3 moles of Na, how many moles of NaCl? Ratio Na:NaCl is 2:2 (or 1:1). So 3 moles Na → 3 moles NaCl. Scenario 2 (Different ratio): In reaction 2H₂+O₂→2H₂O, if 6 moles of O₂ react, how many moles of H₂O? Ratio O₂:H₂O is 1:2. So 6 moles O₂ × (2 moles H₂O/1 mole O₂) = 12 moles H₂O. Scenario 3 (Complex ratio): In reaction CH₄+2O₂→CO₂+2H₂O, if 5 moles of CH₄ react, how many moles of O₂ needed? Ratio CH₄:O₂ is 1:2. So 5 moles CH₄ × (2 moles O₂/1 mole CH₄) = 10 moles O₂. Always set up a proportion: (moles given) × (ratio factor) = moles sought.

The ratio factor comes directly from the equation's coefficients.

Problem 1: In 2H₂+O₂→2H₂O, if 10 moles of H₂ react, how many moles of H₂O form? Ratio H₂:H₂O is 2:2 (1:1). Answer: 10 moles H₂O. Problem 2: In N₂+3H₂→2NH₃, if 5 moles of N₂ react, how many moles of H₂ are needed? Ratio N₂:H₂ is 1:3. Answer: 5 × 3 = 15 moles H₂. Problem 3: In 2C+O₂→2CO, if 8 moles of C react, how many moles of CO form? Ratio C:CO is 2:2 (1:1). Answer: 8 moles CO. Complete all three daily until you quickly set up and solve mole ratio problems.

Verify: Check that units cancel (moles given × moles wanted/moles given = moles wanted).

Stoichiometry questions appear in 1-2 ACT Science chemistry passages. They test application of mole ratios, which is mechanical once you understand the concept. Investing 20 minutes in stoichiometry yields 1-2 guaranteed points because the logic is straightforward and the method is consistent.

Master mole ratios one day before the test. By test day, stoichiometry becomes routine calculation.