A solution has molarity 2.5 M and density 1.25 g mL−1. The molar mass of solute is 100 g mol−1. What is the mass percent of the solution?

Correct answer: 20 percent

Use a 1 L sample because molarity directly gives the solute moles in that volume. Solute moles = 2.5 mol, so solute mass = 2.5 × 100 = 250 g. Density gives solution mass = 1.25 × 1000 = 1250 g. Therefore mass percent = (250/1250) × 100 = 20%, making option C correct. Molarity alone would not provide solution mass; the density is needed for the denominator.