Thermodynamics of solutions:\nWhen the temperature of an aqueous solution is increased (no mass is added or removed), which concentration measure decreases as a result of thermal expansion?

Introduction / Context:
Molarity depends on the solution volume, while mass-based concentration measures do not. Heating an aqueous solution expands the liquid, changing its volume but not the number of moles present. This question tests whether you can distinguish volume-dependent from mass-dependent concentration units under temperature change.

Given Data / Assumptions:

• No evaporation or mass loss; only temperature increases.
• Water and dissolved solute are treated as incompressible over small pressure changes but expand with temperature.
• Solution remains single phase and well mixed.

Concept / Approach:
Molarity M = moles of solute / litres of solution, so it varies inversely with solution volume. Weight percent and mole fraction depend on ratios of masses or moles, which do not change with simple heating. Molality m = moles of solute per kilogram of solvent is mass based and therefore temperature independent (neglecting tiny buoyancy effects).

Step-by-Step Solution:
Identify which quantities depend on Vsolution: only molarity (and normality/formality) do.Upon heating: Vsolution increases due to thermal expansion.Since nsolute constant, M = n/V decreases.Mass- or mole-based measures (wt %, x, molality) remain unchanged.

Verification / Alternative check:
A quick calculation: if volume rises by 1%, molarity falls by ~1% for fixed moles, confirming the trend.

Why Other Options Are Wrong:
Weight percent and mole fraction use mass or mole ratios, unaffected by expansion. Molality uses kilograms of solvent, also mass based. Formality behaves like molarity and would also decrease, but it is not the standard unit in this context.

Common Pitfalls:
Confusing molality with molarity; remembering “molality = mass based, temperature independent” helps avoid this error.

Final Answer:
molarity