NCERT Solutions for Class 9th Science Chapter 5 .2.2 How do we express concentration? — Pause and Ponder

Book page 765 Updated on2026-09-08

Q1.

A common talcum powder contains 4 % m/m zinc oxide, which acts as an antiseptic. How much zinc oxide is present in 300 g of the talcum powder?

Answer

12 g of zinc oxide.

% m/m = (mass of solute ÷ mass of mixture) × 100
4 = (mass of ZnO ÷ 300 g) × 100
mass of ZnO = (4 × 300 g) ÷ 100
mass of ZnO = 12 g

Why it works: “4 % m/m” is a statement about every 100 g of the powder: each 100 g contains 4 g of zinc oxide. 300 g is three such lots, so it contains 3 × 4 g = 12 g. The remaining 300 g − 12 g = 288 g is talc and the other ingredients.

Tip: Notice that mass by mass percentage is used here even though talcum powder is a heterogeneous solid mixture, not a solution. The same formula labels milk powder, spice mixes and packaged foods.

Q2.

Your mother gives you a bottle of orange juice concentrate to mix with water and serve it to your visiting friends. She asks you to mix two tablespoons of the concentrate with water in a glass tumbler. If each tablespoon measures 15 mL and you make 150 mL of juice per person, what is the % v/v of orange juice concentrate in the mixture you prepared?

Answer

20 % v/v.

Volume of concentrate (solute) = 2 × 15 mL = 30 mL
Volume of juice (solution) = 150 mL
% v/v = (volume of solute ÷ volume of solution) × 100
% v/v = (30 mL ÷ 150 mL) × 100
% v/v = 20 % v/v

Why v/v and not m/m here: Both the concentrate and the water are liquids, and in the kitchen it is far easier to measure a liquid by volume (a tablespoon, a measuring cup) than to weigh it. That is exactly the situation the volume by volume percentage was designed for — the same reason vinegar is labelled 5 % v/v acetic acid.

Check it yourself: The water added is 150 mL − 30 mL = 120 mL, so the concentrate and water are in the ratio 30 : 120 = 1 : 4. One part concentrate in five parts of juice is one-fifth, and one-fifth of 100 is 20 — the same answer.

Q3.

Vinegar, used as a food preservative and additive, contains 5 % v/v acetic acid. Glacial acetic acid is a liquid, i.e., 100% acetic acid. If you want to make vinegar from glacial acetic acid, how would you proceed?

Answer

Measure 5 mL of glacial acetic acid and add water to it until the total volume of the solution is 100 mL. That gives 5 % v/v vinegar.

Required: % v/v = 5, volume of solution = 100 mL
5 = (volume of acetic acid ÷ 100 mL) × 100
volume of acetic acid = (5 × 100 mL) ÷ 100 = 5 mL
Water to be added ≈ 100 mL − 5 mL = 95 mL

For a larger batch use the dilution relation, since the amount of pure acetic acid does not change on adding water:

C₁V₁ = C₂V₂
100 % × V₁ = 5 % × 1000 mL
V₁ = 50 mL of glacial acetic acid, made up to 1000 mL with water

Safety first: Glacial acetic acid is corrosive and its vapour stings the eyes. Always add the acid to the water, never water to the acid, do it slowly with stirring, and work under an adult’s supervision. Make the volume up to 100 mL at the end — do not simply add 100 mL of water to 5 mL of acid, because volumes of mixed liquids do not add up exactly.

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