Mass Percent Calculator

How it works

Mass percent (% w/w) expresses the mass of solute as a percentage of the total mass of the solution. It is one of the most common concentration expressions in chemistry and is independent of temperature, unlike molarity, which changes with volume.

The formula

The result also tells you how many grams of solute are present per 100 g of solution. A 5% w/w solution contains 5 g of solute in every 100 g of solution.

Why this works: Because both solute and solution are measured by mass, the ratio is constant regardless of temperature or pressure. This makes % w/w the preferred concentration unit for solid mixtures and situations where temperature variation is expected.

Worked examples

When to use this

• A-level and university chemistry: Mass percent is a standard concentration expression in A-level and first-year university chemistry, particularly for solid solutes.
• Pharmaceutical formulation: Drug concentrations in creams, ointments and some solutions are often specified as % w/w on UK product labelling.
• Industrial chemistry: Concentrated acids (sulphuric, hydrochloric, nitric) are sold with their purity specified as % w/w on safety data sheets.
• Food science: Ingredient concentrations in food products such as salt, sugar or preservative content are sometimes expressed as % w/w in technical specifications.

Understanding the result

A mass percent of 5% means 5 g of solute per 100 g of solution, or equivalently, 5 g in every 95 g of solvent. Note that a 5% w/w solution is slightly less concentrated than you might intuitively expect: 5 g of solute in 100 g of solution means only 95 g of solvent, not 100 g.

Mass percent is different from molarity (mol/L) and molality (mol/kg). Converting between them requires the molecular weight of the solute and, for molarity, the density of the solution.

Related concepts

➡ For solutions where volume is measured rather than mass, the percentage strength calculator handles % w/v and % v/v concentration expressions. ➡ To find the percentage of water in a substance (important in drying and hydration analysis), the percent water calculator uses mass before and after drying. ➡ For expressing mixture composition in moles rather than mass, the mole percentage calculator finds each component's mole fraction as a percentage.

How to do this in Excel

Put solute mass in A1 and solvent mass in B1. If you already have the total solution mass, replace the denominator: =(A1/B1)*100 where B1 is total solution mass.

How to do this without a calculator

Add solute and solvent masses to get the solution mass. Divide solute by solution and multiply by 100. For a 10 g solute in 90 g solvent: 10 ÷ 100 × 100 = 10%. For a 5 g solute in 95 g solvent: 5 ÷ 100 × 100 = 5%.

Common mistakes

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