20 g of common salt is dissolved in 180 g of water. What is the mass percentage of the salt in the solution?
- A.5%
- B.9%
- C.10%
- D.15%
Correct answer
Explanation
The correct answer is C, 10%. The mass of the solution is 20 g + 180 g = 200 g, and 20 g of salt in 200 g of solution is 10 per cent.
Mass percentage of a solute is the mass of the solute divided by the mass of the whole solution, multiplied by 100. The key point is that the solution means solute plus solvent together, not the solvent alone. Here the solute is 20 g of common salt, sodium chloride, and the solvent is 180 g of water, so the solution weighs 200 g. Hence (20 ÷ 200) × 100 = 10 per cent. Concentration can also be stated as mass by volume or volume by volume, but this question asks for mass by mass.
A is wrong because 5 per cent would need only 10 g of salt in a 200 g solution. B is wrong because it does not match either way of working the figures, since 20 ÷ 180 gives about 11.1 per cent, not 9. D is wrong because 15 per cent would need 30 g of salt in 200 g of solution.
Exam tip: Mass percent = solute ÷ (solute + solvent) × 100; never divide by the solvent alone.
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