Question 4 — Solve the following problems - Questions at the end of the chapter - Chapter 5 - Exploring Mixtures and their Separation (Exploration)

Last updated at August 17, 2026 by

Q 4
Question

Solve the following problems:

(i) A cake recipe uses dry ingredients, namely 75 g of sugar for 420 g of all-purpose flour and 5 g of sodium hydrogencarbonate. Express the concentration of each component in the mixture using an appropriate method.

(ii) A brass alloy contains 70% copper by mass. Calculate the quantities of copper and zinc present in 120 g of brass.

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Answer
(i) 15 % sugar, 84 % flour, 1 % sodium hydrogencarbonate.  •  (ii) 84 g copper and 36 g zinc.

Part (i)

Given: all three are dry solids with masses in grams. Method: since everything is a mass, use mass by mass percentage.

Total mass = 75 + 420 + 5 = 500 g Sugar = 75 500 × 100 = 15 % m/m All-purpose flour = 420 500 × 100 = 84 % m/m Sodium hydrogencarbonate = 5 500 × 100 = 1 % m/m

Check: 15 + 84 + 1 = 100. So in every 100 g of the dry mixture there are 15 g of sugar, 84 g of flour and 1 g of sodium hydrogencarbonate.

Part (ii)

Given: 70 % copper by mass in 120 g of brass. To find: the mass of each metal.

Copper = 70 100 × 120 = 84 g Zinc = 120 − 84 = 36 g
Percentages of a mixture must add up to 100 — that is always the check.
← Back to: 5.2.2 How do we express concentration?
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