By Sean Lancaster, Updated Aug 30 2022

In any chemical synthesis, the purity of the final product hinges on how much of the starting materials remain unreacted. Determining the expected yield requires identifying the limiting reactant and calculating the mass of excess reactants that persist in the product.

Step 1: List the Reactants

Write down every substance that participates in the balanced equation. These are the only species that can remain in the finished product.

Step 2: Compute Molecular Weights

Sum the atomic weights of each element in a reactant. For example:

• HCl: 1.008 + 35.453 = 36.461 g mol‑1
• CaCO₃: 40.078 + 12.011 + 3×15.999 = 100.086 g mol‑1

Step 3: Determine the Mole Ratios

From the balanced equation you can read how many moles of each reactant are needed. For the reaction CaCO₃ + 2 HCl → CaCl₂ + CO₂ + H₂O:

• 1 mol CaCO₃ requires 2 mol HCl (ratio 1:2)
• 1 mol HCl requires ½ mol CaCO₃ (ratio 1:½)

Step 4: Identify the Limiting Reagent

Convert the initial masses to moles (mass ÷ molecular weight). In our example:

• 30.027 g CaCO₃ ÷ 100.086 g mol‑1 = 0.300 mol
• 10.938 g HCl ÷ 36.461 g mol‑1 = 0.478 mol

Using the 1:2 ratio, 0.300 mol CaCO₃ would consume 0.600 mol HCl. Because only 0.478 mol HCl is present, HCl is the limiting reagent.

Step 5: Calculate Excess Reactant

Determine how much of the non‑limiting reactant remains after the reaction. 0.478 mol HCl needs 0.239 mol CaCO₃ to react completely. Excess CaCO₃ = 0.300 – 0.239 = 0.061 mol.

Step 6: Convert Excess to Grams

Multiply the excess moles by the molecular weight: 0.061 mol × 100.086 g mol‑1 ≈ 6.10 g CaCO₃ remain in the product.

Tools You’ll Need

• Periodic table (for atomic weights)
• Balanced chemical equation
• Scientific calculator

By following these steps you can accurately quantify unreacted species, enabling you to design purifications that achieve the desired product purity.