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
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
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
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.