How to Calculate Mass Percent Concentration

Finds the mass percent as solute ÷ (solute + solvent) × 100. The denominator is the mass of the whole solution, not of the solvent alone.

Explanation

Mass percent says what fraction of a solution's total mass is solute.

w = \dfrac{\text{mass of solute}}{\text{mass of solute} + \text{mass of solvent}} \times 100

The denominator, solute plus solvent, is the mass of the whole solution.

Example

Dissolve 20 g of salt in 80 g of water.

w = \dfrac{20}{20 + 80} \times 100 = \dfrac{20}{100} \times 100 = 20\ \%

The result is a 20% salt solution.

The classic mistake

Do not divide by the solvent. Writing $\dfrac{20}{80} \times 100 = 25\ \%$ is the error everyone makes once.

A concentration asks what share of the *whole* is solute, so the denominator must be the entire solution: 20 + 80 = 100 g.

Where it is used

Labels on rubbing alcohol (70%) and hydrogen peroxide (3%)
Saline solution for medical use is 0.9%
Seawater is roughly 3.5%

Against molar concentration

The great virtue of mass percent is that a balance is all you need. Weigh out 20 g of solute and 80 g of water, mix, and you have 20%. No volumetric glassware, no molar masses.

It is poor at describing reactions, though. Reactions combine particles in fixed ratios, so those calculations want molar concentration.