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Question
Three students, A, B and C, are preparing sugar solutions for an experiment:
- Student A dissolves 20 g of sugar in 80 g of water.
- Student B dissolves 20 g of sugar in 100 g of water.
- Student C dissolves 30 g of sugar in 80 g of water.
(i)
Calculate the mass percentage (% m/m) concentration of sugar in each student’s solution.
(ii)
Whose solution is the most concentrated? Explain why.
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Answer
A = 20 %, B ≈ 16.7 %, C ≈ 27.3 % m/m.
Student C
has the most concentrated solution.
Explanation
Remember that the denominator is the mass of the solution , not of the water alone.
A: solution = 20 + 80 = 100 g
=
20
100
× 100 =
20 % m/m
B: solution = 20 + 100 = 120 g
=
20
120
× 100 ≈
16.7 % m/m
C: solution = 30 + 80 = 110 g
=
30
110
× 100 ≈
27.3 % m/m
Student C used the most sugar and the least water, so every 100 g of her solution carries the most sugar — about 27.3 g.
Looking at the sugar alone is not enough. A and B both used 20 g, yet A is more concentrated because A used less water.
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