By Kylene Arnold – Updated Aug 30, 2022

TL;DR

To determine moles in a reaction, convert all reactant masses to grams, calculate each compound’s molar mass, divide grams by molar mass, and use the stoichiometric coefficients to find the limiting reagent and product yields.

1. Convert Reactant Masses to Grams

Begin by expressing every reactant’s mass in grams. For example, 0.05 kg of sodium (Na) equals 50.0 g (0.05 kg × 1 000 g/kg). In the sodium‑chlorine example we have 50.0 g Na and 25.000 g Cl₂.

2. Determine Atomic Weights

Retrieve each element’s atomic weight from the periodic table (expressed in atomic mass units, amu). Na = 22.990 amu, Cl = 35.453 amu.

3. Calculate Molar Masses of Reactants and Products

Sum the atomic weights of all atoms in each compound to obtain its molar mass (g mol⁻¹). For example:

• Na: 22.990 g mol⁻¹
• Cl₂: 2 × 35.453 = 70.506 g mol⁻¹
• NaCl: 22.990 + 35.453 = 58.443 g mol⁻¹

4. Convert Grams to Moles

Divide each reactant’s mass by its molar mass:

• Na: 50.0 g ÷ 22.990 g mol⁻¹ = 2.1749 mol
• Cl₂: 25.000 g ÷ 70.506 g mol⁻¹ = 0.3546 mol

5. Inspect the Reaction Coefficients

The balanced equation is 2 Na + Cl₂ → 2 NaCl. Coefficients indicate the mole ratio: 2 Na : 1 Cl₂ : 2 NaCl.

6. Identify the Limiting Reactant

Calculate how many moles of product each reactant can generate using the stoichiometric ratio.

• Na → 2.1749 mol Na × (2 NaCl / 2 Na) = 2.1749 mol NaCl
• Cl₂ → 0.3546 mol Cl₂ × (2 NaCl / 1 Cl₂) = 0.7092 mol NaCl

The smaller value (0.7092 mol NaCl) indicates that Cl₂ is the limiting reactant.

7. Compute the Maximum Mass of Product

Multiply the moles of product by its molar mass:

• 0.7092 mol NaCl × 58.443 g mol⁻¹ = 41.4 g NaCl

8. Verify the Result

Cross‑check by calculating the mass based on the Na limit: 2.1749 mol NaCl × 58.443 g mol⁻¹ = 127.0 g NaCl. Because the Cl₂‑derived amount is smaller, the reaction will produce only ~41.4 g of NaCl.

9. Calculate Final Moles of Product

Divide the product mass by its molar mass to confirm the moles:

• 41.4 g ÷ 58.443 g mol⁻¹ = 0.709 mol NaCl

Safety Note

Sodium is highly reactive; this experiment should only be performed in a controlled laboratory environment by trained professionals.

For authoritative data on Avogadro’s constant and molar mass calculations, see IUPAC and Wikipedia: Avogadro constant.