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Concentration of solutions – practice for Years 8–9

Sample questions on Concentration of solutions with the right answer and a worked solution, from easy to hard. In the Pickles Academy app a child gets new questions of the same kind every time, so they can't just learn the answers by heart.

Subject
Chemistry
Year
Years 8–9
Curriculum area
Mixtures

Question types: How concentrated is the solution?

Rule: Concentration of solutions

mass fraction = mass of solute ÷ mass of solution; solution = solute + water. As a percentage: × 100 %.

Mass of solute = fraction × mass of solution, mass of water = mass of solution − mass of solute.

Example: 250 g of a 2% sugar solution: sugar 0.02 × 250 = 5 g, water 250 − 5 = 245 g

Watch out: Divide by the mass of the whole solution, not just the water: 5 g of sugar in 100 g of water is 5 ÷ 105, not 5%.

Sample questions

  1. Question 1 Easy

    500 g of solution contains 100 g of dissolved salt. What is the percentage by mass of salt?

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    Answer: 20 %

    How it works: w = m(solute) ÷ m(solution) = 100 g ÷ 500 g = 0.2 = 20 %.

  2. Question 2 Normal

    How many grams of water do you need to make 500 g of a 20% salt solution?

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    Answer: 400 g

    How it works: m(solute) = 0.2 × 500 g = 100 g; m(water) = 500 g − 100 g = 400 g.

  3. Question 3 Hard

    100 g of water is added to 300 g of a 30% salt solution. What is the percentage by mass of salt now?

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    Answer: 22.5 %

    How it works: m(solute) = 0.3 × 300 g = 90 g; w = 90 g ÷ 400 g = 22.5 %.

Related topics

All Chemistry topics

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