Mass Percent Formula

Mass percent (also called weight percent or percent composition) expresses the mass of one component as a percentage of the total mass. It's essential for solutions, mixtures, and chemical composition calculations.

Mass Percent Formula

Mass % = (masspart / masstotal) × 100%

Percentage by mass of a component in a mixture

Variable Definitions

Mass % = Mass Percent

Units: % (percent) - dimensionless

The percentage composition by mass

💡 Also called: weight percent, w/w%, percent composition

masspart = Mass of Component

Units: g, kg, mg (any mass unit)

Mass of the specific component you're interested in

masstotal = Total Mass

Units: Same as masspart

Total mass of the entire mixture or solution

For solutions: masstotal = masssolute + masssolvent

Two Main Applications

1. Solution Concentration

% = (msolute / msolution) × 100%

Example: 10% NaCl solution = 10 g salt per 100 g solution

2. Percent Composition

% = (melement / mcompound) × 100%

Example: H₂O has 11.2% H and 88.8% O by mass

Rearranged Forms

Find Mass %:

% = (mpart / mtotal) × 100

Find mpart:

mpart = (% × mtotal) / 100

Find mtotal:

mtotal = (mpart × 100) / %

Example 1: Solution Concentration

Problem: You dissolve 15.0 g of sugar in 135.0 g of water. What is the mass percent of sugar?

Given:

  • masssugar = 15.0 g (solute)
  • masswater = 135.0 g (solvent)
  • Find: Mass % sugar

Step 1: Calculate total mass

masstotal = masssugar + masswater
masstotal = 15.0 g + 135.0 g = 150.0 g

Step 2: Apply mass percent formula

Mass % = (masssugar / masstotal) × 100%

Step 3: Substitute and calculate

Mass % = (15.0 g / 150.0 g) × 100%
Mass % = 0.100 × 100% = 10.0%

Answer: 10.0% sugar solution

Example 2: Percent Composition

Problem: Find the mass percent of oxygen in H₂O. (Molar mass: H = 1.01 g/mol, O = 16.00 g/mol)

Step 1: Calculate molar mass of H₂O

M(H₂O) = 2(1.01) + 16.00 = 18.02 g/mol

Step 2: Mass of oxygen in 1 mole H₂O

massO = 16.00 g (in 1 mol H₂O)

Step 3: Apply formula

Mass % O = (16.00 / 18.02) × 100%
Mass % O = 88.8%

Answer: Water is 88.8% oxygen by mass (and 11.2% hydrogen)

Common Mistakes

❌ Using mass of solvent instead of total mass

Denominator must be TOTAL mass (solute + solvent), not just solvent mass!

❌ Forgetting to multiply by 100

Mass percent needs × 100 to convert from decimal to percent. 0.25 = 25%, not 0.25%!

❌ Unit mismatches

masspart and masstotal must use the same units. Both in grams, or both in kg, etc.

❌ Confusing with molarity

Mass percent is mass/mass. Molarity is moles/volume. Completely different concentration units!

Related Calculators

Mass Percent Calculator

Calculate mass %, mass part, or mass total

Percent Composition

Calculate % of each element

Concentration Converter

Convert between units

Solution Preparation

Prepare % solutions

Concentration & Solution Calculators

All concentration tools

Mass Percent vs Mole Fraction

Compare composition units

Frequently Asked Questions

What is mass percent?

Mass % = (mass of part / total mass) × 100%. It tells you what percentage of the total mass is made up by a specific component.

How do I make a 5% solution?

5% means 5 g solute per 100 g total solution. Dissolve 5 g solute in 95 g solvent to get 100 g of 5% solution.

Is mass percent the same as weight percent?

Yes! They're identical. "Weight percent" is technically incorrect (should be mass), but it's commonly used and means the same thing.

Can mass percent be over 100%?

No! A part can't be larger than the whole. If you get over 100%, you made a calculation error (probably used wrong total mass).

How is this different from volume percent?

Mass % = mass/mass × 100. Volume % = volume/volume × 100. Use mass % for solids in liquids, volume % for liquids in liquids (like alcohol content).

← Back to All FormulasNext: Mole Fraction →