Quick answer: Mass percent tells you what share of a solution’s total mass comes from one component. The formula is mass percent = (mass of the component ÷ total mass of the solution) × 100. For example, 10 g of glucose dissolved in 90 g of water makes 100 g of solution, so the solution is 10% glucose by mass. Always divide by the mass of the whole solution, not the mass of the solvent.
The mass percent formula is one of the basic ways chemists express the concentration of a solution, that is, how much of a substance is present in a mixture. It is also called mass percentage, weight percent or w/w %, and it appears in the National Council of Educational Research and Training (NCERT) chemistry textbooks for both Class 11 and Class 12.
Solute, solvent and solution
Before using the formula, it helps to be clear about three terms, as NCERT’s Class 12 chapter on solutions defines them:
- Solution: a homogeneous mixture of two or more components, with the same composition and properties throughout.
- Solvent: generally the component present in the largest quantity. It decides the physical state of the solution.
- Solute: any component other than the solvent. A solution can have one solute or several.
An earlier version of this article said that a solution is “a mixture of solutes”. That is not quite right: a solution is made of one or more solutes and a solvent.
The mass percent formula
The Mass Percent Formula can be stated as:
Mass percent of a component = (mass of the component in the solution ÷ total mass of the solution) × 100
Here, the total mass of the solution is the mass of the solute (or solutes) plus the mass of the solvent. Both masses must be in the same unit, usually grams, and the answer is a percentage.
Because it uses masses only, mass percent does not change with temperature. NCERT notes that mass percent, parts per million, mole fraction and molality are independent of temperature, while molarity is not, because volume changes with temperature and mass does not.
Worked examples
Example 1: salt in a solution
A 50 g salt solution contains 8 g of dissolved salt. Mass percent of salt = (8 g ÷ 50 g) × 100 = 16%.
Example 2: ethanol in water
A mixture contains 60 g of ethanol (solute) dissolved in 240 g of water (solvent). First find the total mass: 60 g + 240 g = 300 g. Mass percent of ethanol = (60 g ÷ 300 g) × 100 = 20%. Dividing by 240 g, the mass of water alone, would give 25%, which is a common mistake.
Example 3: carbon dioxide in a gas mixture
A gas mixture contains 20 g of carbon dioxide and 80 g of oxygen. Oxygen is present in the larger amount, so it acts as the solvent. Mass percent of carbon dioxide = (20 g ÷ 100 g) × 100 = 20%.
Example 4: benzene in carbon tetrachloride
This one is from the NCERT Class 12 textbook: 22 g of benzene is dissolved in 122 g of carbon tetrachloride. The total mass is 144 g. Mass percent of benzene = (22 ÷ 144) × 100 ≈ 15.28%, and mass percent of carbon tetrachloride = (122 ÷ 144) × 100 ≈ 84.72%. The two add up to 100%, which is a quick way to check your answer.
Example 5: working backwards to make a solution
To make 250 g of a 5% (w/w) salt solution, you need 5% of 250 g = 12.5 g of salt. The rest, 250 g − 12.5 g = 237.5 g, is water.
Mass percent of an element in a compound
The same idea is used for a different purpose in Class 11: finding the percentage composition of a compound. NCERT gives the formula as:
Mass percent of an element = (mass of that element in the compound ÷ molar mass of the compound) × 100
For ethanol (C2H5OH), NCERT calculates the molar mass as 46.068 g, which gives about 52.14% carbon, 13.13% hydrogen and 34.73% oxygen by mass. Percentage composition is used to work out a compound’s empirical formula and to check the purity of a sample.
The original version of this article said the mass percent formula is used “to solve the molar mass” of each element. That mixed up the two uses: the formula does not give you molar mass; it uses the molar mass to find each element’s share of the compound.
Mass percent compared with other percentage units
NCERT lists three related percentage measures. They look similar but are not interchangeable:
| Measure | What it compares | Typical use |
|---|---|---|
| Mass percentage (w/w) | Mass of component per 100 g of solution | Industrial chemicals, such as commercial bleach described by NCERT as 3.62% sodium hypochlorite by mass |
| Volume percentage (V/V) | Volume of component per 100 mL of solution | Mixtures of liquids, such as a 35% (v/v) ethylene glycol antifreeze |
| Mass by volume percentage (w/V) | Mass of solute in 100 mL of solution | Medicine and pharmacy |
For very small amounts, chemists use parts per million (ppm) instead of percentages.
Why mass percent matters
Mass percent is a simple, practical way to describe concentration. It is used in chemical industries, in quality checks that compare a sample with a pure standard, in pharmaceutical and environmental work, and in laboratory recipes for preparing solutions. In the CBSE syllabus for 2026–27, expressing concentration appears in Class 11 (“Some Basic Concepts of Chemistry”, including percentage composition) and in Class 12 (“Solutions”, including the expression of concentration of solutions).
For more science learning resources, see our list of technology tools for science classrooms. Class 12 students can also check the important chapters in CBSE Class 12 Maths, and those preparing for engineering entrance exams can read the best study materials for JEE.
Frequently asked questions
What is the difference between mass percent and mass by volume percent?
Mass percent compares the mass of a component with the total mass of the solution. Mass by volume percent gives the mass of solute in 100 mL of solution, so it mixes a mass with a volume.
Do I divide by the mass of the solvent or the mass of the solution?
The mass of the solution: solute plus solvent. Dividing by the solvent alone gives a wrong, larger answer.
Can mass percent be more than 100%?
No. A component cannot weigh more than the whole solution, so a single component’s mass percent is always between 0 and 100, and all the components add up to 100%.
Does mass percent change with temperature?
No. Mass does not change with temperature, so mass percent stays the same. Molarity, which depends on volume, does change.
This explainer was checked in October 2026 against the NCERT Chemistry textbooks for Class 11 (Unit 1, Some Basic Concepts of Chemistry) and Class 12 (Unit 1, Solutions) and CBSE’s Chemistry syllabus for 2026–27.
