A solution contains 20 g of solute in 180 g of solvent. If the solvent is water, what is the concentration of the solution in terms of mass by mass percentage?
- A.11.1%
- B.22.2%
- C.10%
- D.20%
Correct answer
Explanation
The correct answer is C, 10%. Mass by mass percentage is the mass of solute divided by the mass of the whole solution: 20 ÷ (20 + 180) × 100 = 10%.
The concentration of a solution tells how much solute is present in a given amount of solution, not in the solvent alone. Here the solution weighs 20 g plus 180 g, that is 200 g, and 20 g of it is solute, so the solute makes up one tenth of the total. The same idea gives mass by volume percentage, where the mass of solute is divided by the volume of the solution.
A is wrong because 11.1% comes from dividing 20 g by the 180 g of solvent instead of by the 200 g of solution. B is wrong because 22.2% has no basis in these figures; it is double the wrong value in A. D is wrong because 20% would need 20 g of solute in only 100 g of solution.
Exam tip: Mass % = mass of solute ÷ mass of solution × 100; always add the solute to the solvent first.
Practice Questions
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