An easy question on percentages and mixtures for P5, with a step-by-step solution, diagrams and the answer. Common in the Hua Xia Cup's P6 first round and finals: discounts, a rise followed by a fall, and mixtures.
The question
In a science lesson, Ming stirs 18 g of salt into 432 g of water until it all dissolves.
The concentration of salt water is the weight of the salt as a percentage of the weight of the whole solution.
What is the concentration of this salt water, as a percentage?
Show the hints (2)
💡Hint 1: Concentration compares the salt with the whole solution. How many grams is the whole solution?
💡Hint 2: The whole solution = salt + water. What percentage of it is 18 g?
Worked solution
Have a go first, then read the solution step by step.
1.Weight of the solution = salt + water = 18 + 432 = 450 g.
2.Concentration = 18 ÷ 450 × 100% = 4%.
⚠️Common mistake: working out 18 ÷ 432. Divide by the whole solution (salt and water together), not by the water alone.
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