Concentration of a solution

A solution has two parts. The solute is the substance that dissolves, such as salt. The solvent is what it dissolves in, such as water. Salt plus water together make the solution.

Two simple ways to state concentration:

• Mass by mass percentage = mass of solute ÷ mass of solution × 100.
• Mass by volume percentage = mass of solute ÷ volume of solution × 100, with mass in g and volume in mL.

A small example

20 g of salt is dissolved in 180 g of water. Find the mass by mass percentage.

• Mass of solution = 20 + 180 = 200 g.
• Concentration = 20 ÷ 200 × 100 = 10%.

The step that loses the answer is the bottom of the fraction. Dividing by the water alone gives 20 ÷ 180 × 100, about 11.1%. That is wrong, because the rule uses the whole solution.

Now a mass by volume case. 10 g of sugar is dissolved to make 250 mL of solution. The concentration is 10 ÷ 250 × 100 = 4%. Always check which form a question asks for.

Where it fits

This goes beyond the Class 10 board syllabus. HBCSE's Junior Science syllabus lists it under the nature of matter. That syllabus is for NSEJS, which the Indian Association of Physics Teachers (IAPT) conducts.

About the author

Deepti Sardana

Founder Member & Academic Lead

An IIT alumna (M.Sc., M.Phil.) with over 25 years in teaching, child psychology and learning assessment. As Academic Lead at EduBrahma, she leads the method TRUtesting uses to find where and why a child loses marks.

Deepti Sardana on LinkedIn