Mass Percent Composition

Mass percent = (mass of solute ÷ mass of solution) × 100%: a simple parts-per-hundred concentration.

\%\text{ mass} = \frac{m_{\text{solute}}}{m_{\text{solution}}} \times 100\%

mass percent = (mass of solute / mass of solution) x 100%

What it means

Mass percent tells you how many grams of solute sit in every 100 grams of solution. It needs no molar masses and no volume measurements, so it is the quickest concentration to prepare and to read on a label. It is especially common for concentrated acids and bases sold by the bottle, where the label quotes the mass percent straight from the manufacturer.

Variables

Symbol	Name	Unit	Meaning
$m_solute$	Mass of solute	g	The mass of the dissolved substance.
$m_solution$	Mass of solution	g	The mass of the entire solution: solute plus solvent.
$% mass$	Mass percent	%	The percentage result: grams of solute per 100 grams of solution.

Assumptions

Both masses use the same units so the units cancel.
Solution mass equals solute mass plus solvent mass.
Mass percent is sometimes called weight percent.
The solute is fully dissolved and uniformly distributed.

Worked example

Problem

A solution is prepared by dissolving 15.0 g of NaCl in 85.0 g of water. What is the mass percent of NaCl?

Given:

Mass of NaCl = 15.0 g
Mass of water = 85.0 g

Find the total solution mass: 15.0 g + 85.0 g = 100.0 g.
Write the ratio: 15.0 g solute / 100.0 g solution.
Divide: 15.0 / 100.0 = 0.150.
Multiply by 100%: 0.150 × 100% = 15.0%.

15.0% NaCl by mass

Every 100 g of this solution carries 15.0 g of salt — a brine-strength concentration, far saltier than seawater.