MCAT General Chemistry Question 128: Answer and Explanation

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Question: 128

8. One hundred grams of sucrose are dissolved in a cup of hot water at 80°C. The cup of water contains 300.00 mL of water. What is the percent composition by mass of sugar in the resulting solution?

(Note: Sucrose = C₁₂H₂₂O₁₁, density of water at )

A. 25.0%
B. 25.5%
C. 33.3%
D. 34.2%

Correct Answer: B

Explanation:

The mass percent of a solute equals the mass of the solute divided by the mass of the total solution times 100%. To find the mass of the solution, first find the mass of the solvent, water. Multiplying the volume of the water by the density gives a mass of 292.5 grams of water. Adding 100 grams of sugar yields a solution with a mass of 392.5 grams. Next, divide 100 grams of sugar by 392.5 grams and multiply by 100 to get 25.5%. Choice (A) results if water's density at 80°C is assumed to be If we had forgotten to add the solute's mass to the solvent's, we would have calculated 34.2%, which is choice (D). Choice (C) neglects both the addition step and the correct density value.

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