By Michael Merry
Updated August 30, 2022

Understanding how much of a reaction actually proceeds is essential in chemistry. When one or more reactants are present in excess, the reaction is limited by the limiting reactant. By converting masses to moles, applying the balanced equation, and identifying the limiting species, you can compute the exact mass of products formed and the total mass that takes part in the reaction.

Step 1 – Write the balanced equation

For a simple zinc‑sulfur system the reaction is:

Zn + S → ZnS

Step 2 – Convert masses to moles

Calculate the amount of each reactant in moles:

• Zinc: 12.00 g ÷ 65.38 g mol⁻¹ = 0.184 mol Zn
• Sulfur: 7.50 g ÷ 32.06 g mol⁻¹ = 0.234 mol S

Step 3 – Identify the limiting reactant

The stoichiometry is 1:1. Because 0.184 mol Zn < 0.234 mol S, zinc is the limiting reactant. All of the zinc will convert to ZnS, while the surplus sulfur remains unreacted.

Step 4 – Determine excess reactant and total mass of reaction

Excess sulfur:

• 0.234 mol S – 0.184 mol Zn = 0.050 mol S
• 0.050 mol S × 32.06 g mol⁻¹ = 1.60 g S (unreacted)

Total mass that actually participates in the reaction:

• 12.00 g Zn + 7.50 g S – 1.60 g S = 17.90 g

Step 5 – Calculate the mass of the product

The mass of ZnS produced equals the mass of reaction:

0.184 mol ZnS × 97.44 g mol⁻¹ = 17.90 g ZnS

Things You’ll Need

• Pen and paper
• Scientific calculator

TL;DR

Molar mass is the mass of one mole of a substance; a mole contains 6.022×10²³ entities. Stoichiometry lets you convert between mass, moles, and number of molecules.