Concentration measures — On heating an aqueous solution, which concentration unit decreases due to thermal expansion of the solution volume?

Introduction / Context:
Chemical engineers frequently choose among several concentration units. Some depend on mass, others depend on volume. Since volume changes with temperature (thermal expansion or contraction), volume-based measures respond to temperature changes while mass-based measures do not. This question identifies which common unit decreases upon heating an aqueous solution.

Given Data / Assumptions:

• Aqueous solution heated (temperature increases).
• Solute amount and solvent amount are fixed (no evaporation or addition).
• Solution expands upon heating (volume increases).

Concept / Approach:

Molarity (mol/L) depends on the solution volume in the denominator. When temperature rises, solution volume typically increases due to thermal expansion, so molarity falls if the amount of solute is fixed. In contrast, molality (mol/kg solvent), mole fraction (moles basis), and weight percent (mass basis) do not involve solution volume; therefore, they remain essentially unchanged with temperature (ignoring tiny mass changes).

Step-by-Step Solution:

Verification / Alternative check:

Laboratory practice: volumetric flasks are calibrated at a reference temperature (often 20°C). Accurate molarity preparations require temperature control; molality and weight percent do not.

Why Other Options Are Wrong:

A/B/C/E: These measures do not include the solution volume explicitly and hence do not systematically decrease when temperature rises.

Common Pitfalls:

Using molarity in reactions with significant temperature change without correcting for density/volume; confusing molarity with molality due to similar names.

Final Answer:

molarity