Find the percent purity of a sample from the mass of pure substance and total sample mass, solve for a missing mass when you already know the purity, or work purity out from a reaction's crude and purified product masses.
Inputs
Mass of the pure substance in the sample, in grams.
Mass of the whole sample (pure substance plus impurities), in grams.
Result
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Enter the pure substance mass and total sample mass to compute the percent purity.
Solve for
The selected field is calculated for you — fill in the other two.
Known purity of the sample, as a percentage (0–100).
Known mass of the pure substance, in grams.
Known mass of the whole sample, in grams.
Result
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Choose what to solve for, then fill in the other two fields.
Reaction product masses
Total mass of the crude (unpurified) product isolated from the reaction, in grams.
Mass of pure product recovered after recrystallization or purification, in grams.
Result
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Enter the crude and purified product masses to compute the percent purity.
4 min read3 steps6 terms3 examples6 FAQs% purity = (pure mass / total mass) × 100
Percent purity tells you what fraction of a sample's mass is the substance you actually want, rather than leftover starting material, byproducts, or other contaminants.
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Walk-through
How to Use This Calculator
3 steps▸
1
Enter the pure and total sample mass
On the Percent Purity tab, enter the mass of the pure substance and the total mass of the sample (pure substance plus impurities), both in grams. The calculator updates instantly as you type.
2
Read the purity and impurity mass
The result card shows the percent purity — pure mass divided by total mass, times 100 — along with the impurity mass (total mass minus pure mass) so you can see both numbers at a glance.
3
Solve for a missing mass, or work from a reaction
If you already know the purity and one mass, switch to the Solve tab and pick what to calculate. If you're purifying a reaction product, use the From Reaction tab to find purity from the crude and purified product masses.
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Reference
Formula & Methodology
2 formulas▸
Percent purity
% purity = (pure mass / total mass) × 100
Percent purity compares the mass of the pure substance in a sample to the total mass of that sample. It works the same whether the sample came straight from a container or is the purified product of a chemical reaction — only the labels for the two masses change.
Impurity mass
impurity mass = total mass − pure mass
Whatever mass in the sample isn't the pure substance is impurity. It always equals the total sample mass minus the pure substance mass, regardless of which value you're solving for.
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Glossary
Key Terms Explained
6 terms▸
Percent purity ↗The proportion of a sample that is the pure substance, expressed as a percentage: (pure mass / total mass) × 100.
Pure substance ↗The single chemical compound or element of interest in a sample, free of any contaminants or byproducts.
Impurity ↗Any material in a sample that is not the pure substance — unreacted starting material, byproducts, solvent residue, or other contaminants. Impurity mass equals total mass minus pure mass.
Sample mass ↗The total measured mass of everything in the sample — pure substance plus impurities — usually found by weighing on a balance.
Assay ↗A laboratory test used to determine the purity or concentration of a substance in a sample, often reported as a percent-purity value.
Yield ↗The amount of product obtained from a chemical reaction, usually expressed as percent yield (actual vs. theoretical moles). Yield measures quantity; purity measures quality — a reaction can have a high yield of an impure product, or a low yield of a very pure one.
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Scenarios
Real-World Examples
3 worked examples▸
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Lab technician
Checking a purified sample
Pure substance mass 8.5 gTotal sample mass 10 g
8.5 g of pure substance out of a 10 g sample gives 8.5 / 10 × 100 = 85% purity, with 1.5 g of impurity. Raising the pure mass toward 10 g (or shrinking the total mass toward 8.5 g) is what would push purity toward 100%.
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Student solving backward
Solving for pure mass from a known purity
Percent purity 85%Total sample mass 10 g
A spec sheet lists 85% purity for a 10 g sample. On the Solve tab, choosing "Pure mass" as the target gives 0.85 × 10 = 8.5 g of pure substance and 1.5 g of impurity — the same numbers as the direct calculation, just worked in reverse.
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Organic chemistry student
Purity after recrystallization
Crude product mass 12 gPurified product mass 9.6 g
A reaction produces 12 g of crude (unpurified) product, and recrystallization recovers 9.6 g of pure product. Purity is 9.6 / 12 × 100 = 80%, with 2.4 g lost to the mother liquor and other impurities — a separate number from the reaction's percent yield.
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Reference
Cite This Calculator
APA & MLA▸
Use either format to cite this calculator in a paper, report, or resource list.
Percent purity tells you what fraction of a sample's mass is the substance you actually want, rather than leftover starting material, byproducts, or other contaminants. It's a routine check in analytical chemistry, quality control, and any synthesis that ends with an isolation or purification step.
How the Percent Purity Calculator works
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The calculator applies one formula in three directions. Given the mass of pure substance and the total sample mass, it computes percent purity directly: (pure mass / total mass) × 100. Given a known purity and one of the two masses, the Solve tab rearranges the same formula to find the missing mass. And on the From Reaction tab, the identical math is reframed around a synthesis: the "total mass" becomes the crude (unpurified) product isolated straight from the reaction, and the "pure mass" becomes what's recovered after a purification step such as recrystallization, distillation, or column chromatography.
In every mode, impurity mass is simply total mass minus pure mass — it's never entered directly, only derived.
Inputs and what they mean
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Pure substance mass and total sample mass should be in the same units (this calculator uses grams). Total sample mass must always be greater than zero — dividing by a zero or negative sample mass isn't physically meaningful, and the calculator flags it rather than returning an error value. Pure mass can't exceed total mass, since impurity mass can't be negative.
On the Solve tab, percent purity must fall between 0% and 100%; when solving for total mass, purity also can't be exactly 0%, since that would imply an infinitely large sample containing no pure substance at all.
Limits and edge cases
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Percent purity by mass is a simple ratio — it doesn't account for how the impurity was measured or identified. In a real lab, purity is usually confirmed with an instrumental method (melting point, HPLC, NMR integration, titration) rather than mass alone, especially when impurities are hard to separate cleanly by weighing. This calculator also doesn't distinguish impurity type; 2 g of unreacted starting material and 2 g of a toxic byproduct both register as "2 g of impurity." And percent purity is a different quantity from percent yield: purity describes how clean a product is, while yield describes how much of it you obtained relative to the theoretical maximum — a synthesis can have a high yield of an impure product, or a disappointing yield of something extremely pure.
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Questions
Frequently Asked Questions
6 questions▸
What is the formula for percent purity?+
Percent purity = (mass of pure substance / total sample mass) × 100. For example, 8.5 g of pure substance in a 10 g sample is 8.5 / 10 × 100 = 85% pure.
How do I find the impurity mass?+
Impurity mass equals total sample mass minus pure substance mass. For an 85%-pure 10 g sample, impurity mass is 10 − 8.5 = 1.5 g.
How do I calculate purity from a reaction?+
Weigh the crude (unpurified) product straight from the reaction, then weigh the purified product after recrystallization or another purification step. Percent purity is (purified mass / crude mass) × 100 — the same formula used for any sample, just applied to a reaction's expected vs. actual product masses.
Is percent purity the same as percent yield?+
No. Percent yield compares the actual moles (or mass) of product obtained to the theoretical maximum from the reaction stoichiometry. Percent purity compares the mass of pure substance to the total mass of whatever you isolated. A reaction can have a high yield of an impure product, or a low yield of a very pure one — they're related but measure different things.
What units does the Percent Purity Calculator use?+
Pure substance mass and total sample mass are entered in grams (any consistent mass unit works, since the calculator only takes their ratio). The result is a unitless percentage, and the impurity mass is reported in the same unit as the inputs.
What is an assay in this context?+
An assay is a laboratory test that measures how much of a specific substance is present in a sample, often reported as a percent-purity figure. This calculator performs the arithmetic once you already have pure and total masses — it doesn't replace the lab assay itself.
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