How to Calculate Percentage Purity Step by Step

Updated 11 November 2026 · Standard formula, shown below

This guide explains how to calculate the percentage purity of a substance, a key concept in chemistry for assessing the quality of reagents, ores, and pharmaceutical compounds. You’ll learn the formula, work through a step-by-step example, and see how to apply it to real lab data. No prior knowledge beyond basic stoichiometry is needed.

What You’ll Need

A balanced chemical equation for the reaction used in the analysis
Mass of the impure sample (in grams)
Mass of the pure product obtained (in grams) or the volume/concentration of a titrant
Molar masses of the relevant compounds (from the periodic table)
A calculator

Step-by-Step

Write the balanced equation for the reaction that produces a measurable product (e.g., a precipitate, gas, or titration endpoint). For example, if you are analyzing calcium carbonate in chalk, the reaction with HCl is: CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂.
Weigh the impure sample accurately. Record this as the “sample mass.” For instance, 2.50 g of impure chalk.
Perform the reaction and isolate the product that corresponds to the pure substance. This could be a dried precipitate, a collected gas, or the amount of titrant consumed. Record the mass or volume.
Convert the product amount to moles. If you have a mass, divide by the molar mass. If you have a titration volume and concentration, use: moles = concentration (mol/L) × volume (L).
Use the stoichiometry of the balanced equation to find the moles of the pure substance in the original sample. For example, if 1 mole of product corresponds to 1 mole of the analyte, the moles are equal.
Convert moles of pure substance to mass by multiplying by its molar mass. This is the “pure mass” in the sample.
Calculate percentage purity using the formula: (pure mass / sample mass) × 100%. Ensure both masses are in the same unit (usually grams).
Round appropriately to the number of significant figures consistent with your measurements (usually 2–3 significant figures).

Common Mistakes

Using the wrong stoichiometric ratio: Forgetting to account for coefficients in the balanced equation. For example, if 2 moles of NaOH react with 1 mole of H₂SO₄, you must divide the moles of NaOH by 2 to get moles of H₂SO₄.
Unit mismatches: Mixing grams and kilograms, or mL and L. Always convert to the same unit before dividing.
Confusing purity with yield: Percentage purity compares the amount of pure substance in the sample to the sample mass, not to the theoretical maximum product. Yield compares actual product to theoretical product.
Ignoring side reactions: If the product is not solely from the pure substance, the calculation will be off.
Rounding too early: Round only at the final step to avoid accumulating errors.

Tips

Always double-check the balanced equation for correct coefficients.
Use a high-precision balance if possible; small errors in sample mass affect the final percentage.
If you are titrating, perform three trials and average the results to reduce random error.
Dry precipitates thoroughly before weighing—residual water adds mass and inflates the purity.
When in doubt, write out the units at each step to ensure they cancel correctly.

Troubleshooting

If your percentage purity is above 100%: This usually indicates that the product is contaminated (e.g., water or other solids) or that the sample lost mass during handling. Check your drying procedure and re-weigh.

If the purity is far below expected: The reaction may be incomplete, or you may have lost product during filtration/transfer. Review the procedure for losses.

If your moles don’t match the stoichiometry: Re-read the balanced equation. Make sure you are using the correct compound’s molar mass.

If you used a titration and the endpoint was unclear: The volume of titrant may be inaccurate. Repeat the titration with a fresh indicator or a more precise burette.

Frequently Asked Questions

What is percentage purity?

It is the mass of the pure substance divided by the mass of the impure sample, multiplied by 100%. It tells you how much of a sample is actually the desired compound.

Can percentage purity be more than 100%?

Not in a correctly performed analysis. Values above 100% indicate experimental error, such as incomplete drying or contamination of the product.

What is the difference between percentage purity and percentage yield?

Purity compares the pure substance in the sample to the sample mass. Yield compares the actual product obtained to the theoretical maximum from the reaction. They are independent measures.

Do I need a balanced equation for every purity calculation?

Yes, if the analysis involves a chemical reaction. The balanced equation gives the mole ratio between the measured product and the pure substance.

Can I calculate purity without a chemical reaction?

Sometimes, if you can physically separate the pure substance (e.g., by evaporation or crystallization) and weigh it directly. Then purity = (mass of pure substance / sample mass) × 100%.

How do I handle hydrates or compounds with water of crystallization?

Include the water molecules in the molar mass of the pure substance when calculating moles. For example, CuSO₄·5H₂O has a molar mass of 249.7 g/mol, not 159.6 g/mol.

Method: the standard formula for this calculation, rounded as each result states. See how the calculators work or report an error.