ACT Science: Find the Limiting Reactant and Predict Yields

Step 1: Convert all given masses to moles using molar mass. Step 2: Divide each mole amount by the coefficient in the balanced equation. The reactant with the smallest result is the limiting reactant. Example: 2H₂+O₂→2H₂O. You have 4 moles H₂ and 3 moles O₂. H₂: 4/2=2. O₂: 3/1=3. H₂ is limiting because 2<3. Always perform the division step; it identifies the limiting reactant every time.

Once you identify the limiting reactant, use its mole amount and the molar ratios to find how much product forms. In the example, 4 moles H₂ produces 4 moles H₂O (1:1 ratio). If you used O₂ instead, you'd get 6 moles H₂O, which is wrong.

Pitfall 1: Assuming the reactant with the larger mass is limiting. Not true; you must convert to moles first. Pitfall 2: Forgetting to use the balanced equation coefficients. The ratio 2H₂:1O₂ matters more than raw mole counts. Pitfall 3: Using the non-limiting reactant to calculate product. This gives an artificially high answer. Always use the limiting reactant's moles to find product yield.

Quick check: If you calculated product using both reactants and got different answers, the smaller value is correct. This confirms which reactant is limiting.

Scenario: 10 grams Fe and 10 grams O₂ react via 4Fe+3O₂→2Fe₂O₃. Find the limiting reactant and product moles. Step 1: Moles Fe=(10 g)/(56 g/mol)≈0.18 mol. Moles O₂=(10 g)/(32 g/mol)=0.31 mol. Step 2: Fe: 0.18/4=0.045. O₂: 0.31/3≈0.10. Fe is limiting. Step 3: 0.18 mol Fe produces (0.18)·(2/4)=0.09 mol Fe₂O₃. Work through this problem twice to lock in the method.

Verify by checking O₂: 0.31 mol O₂ produces (0.31)·(2/3)≈0.21 mol Fe₂O₃. Since 0.09<0.21, Fe is confirmed limiting. Excess O₂ remains.

Limiting reactant questions test whether you understand that reactions stop when one reactant is exhausted, not when all reactants are used. This is a core chemistry concept. Expect 1-2 limiting reactant questions on your ACT Science section, and the mole ratio method answers them both.

Spend 15 minutes drilling 3-4 limiting reactant problems this week. By test day, the method will be so automatic that you'll solve these questions in under two minutes and move on to harder content.