Percent Purity Calculator: From Mass, a Reaction Product or a Titration
Calculate the percentage purity of a sample from the mass of pure substance it contains, from the mass of a product it forms in a reaction (such as CO₂ from a carbonate), or from a titration, with impurity %, impurity mass, ppm and the sample mass needed for a set amount of pure substance.
How it works
Purity (%) = mass of pure substance ÷ total mass of sample × 100 · From a product: pure mass = (product mass ÷ product molar mass) × mole ratio × molar mass of the substance · From a titration: pure mass = concentration × titre (L) × dilution factor × mole ratio × molar mass · Impurity (%) = 100 − purity · Sample needed = pure mass needed ÷ (purity ÷ 100)
Percent purity tells you what share of a sample, by mass, is the substance you want; the rest is impurities. It is the mass of the pure component divided by the total mass of the sample, times 100. The pure mass usually comes from an analysis. In the reaction method, the sample reacts completely and you measure one product, such as the carbon dioxide given off when limestone reacts with acid or a dried precipitate. Moles of product times the mole ratio from the balanced equation give moles of the pure substance, and multiplying by its molar mass gives grams. The titration method does the same with the moles of titrant (concentration × volume); if you titrated only part of a solution, the dilution factor scales the result back to the whole sample. These methods assume that only the substance of interest reacts, that the reaction is complete and that nothing is lost. A result above 100% points to a measurement or set-up error, such as a wet precipitate, a wrong mole ratio or impurities that also react.
Worked example
A 10 g sample of impure salt is found to contain 8.5 g of pure NaCl. Purity = 8.5 ÷ 10 × 100 = 85.00%, so 15.00% of the sample, 1.5 g, is impurities. With the reaction method, 4 g of impure CaCO₃ treated with excess acid gives 0.88 g of CO₂: 0.88 ÷ 44 = 0.02 mol CO₂ = 0.02 mol CaCO₃ (1:1), which is 0.02 × 100 = 2 g of CaCO₃, so the sample is 2 ÷ 4 × 100 = 50% pure.
Sources: Careers360: purity of CaCO₃ from CO₂ produced (4 g sample, 0.88 g CO₂) · NNU: impure calcium carbonate dissolved in HCl (purity from CO₂ produced) · IUPAC CIAAW: Abridged standard atomic weights (molar masses)
Frequently asked questions
How do you calculate percent purity?
Divide the mass of the pure substance by the total mass of the sample and multiply by 100. A 10 g sample containing 8.5 g of the pure compound is 85% pure.
How do I find the mass of the pure substance?
Usually from an analysis: a titration, a gas or precipitate produced in a reaction, or a known yield. Convert the measured product or titrant to moles, multiply by the mole ratio from the balanced equation and then by the molar mass of the substance.
How do I calculate purity from a titration?
Moles of titrant = concentration × titre in litres. Multiply by the mole ratio to get moles of the substance, then by its molar mass for grams, and divide by the sample mass. If you made up the sample to 250 mL and titrated 25 mL, multiply the moles by 10 first.
Can purity be over 100%?
No. A value above 100% means a measurement or calculation error, commonly a pure mass entered larger than the sample mass, a wrong mole ratio, a precipitate that was not fully dry, or impurities that also reacted.
How is purity different from percent yield?
Purity describes the composition of one sample. Percent yield compares how much product a reaction made with the theoretical maximum. An impure product makes the yield look higher than it really is unless you correct for purity.
How much impure material do I need for a given amount of pure substance?
Divide the pure mass you need by the purity as a decimal. To get 50 g of NaCl from 85% pure salt, weigh 50 ÷ 0.85 = 58.82 g. Enter the amount under More options to have it calculated.
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Educational estimate. Check the balanced equation, units and significant figures for your exact case.