Question

Molality is preferred over molarity in calculations involving temperature changes because:

• A. It depends on volume
• B. It is independent of temperature
• C. It depends on pressure
• D. It changes with density

Hint:

Logic Tip: Any concentration term that includes "Volume" (Molarity, Normality, % v/v) will change with temperature. Any term that uses only "Mass" or "Moles" (Molality, Mole Fraction, % w/w) is perfectly temperature-independent!

Answer 1

The Correct Option is B

Concept:
When conducting experiments that involve heating or cooling a solution (such as boiling point elevation or freezing point depression), the concentration metric used must not artificially change merely because the temperature changes.

Step 1:
Molarity is defined as moles of solute per Liter of solution. Because liquids expand when heated and contract when cooled, the volume (Liters) of the solution fluctuates with temperature.

Step 2:
Since the denominator (volume) changes with temperature, the calculated Molarity value itself will change as the temperature shifts, even if no solute is added or removed. This makes it an unreliable metric for thermal experiments.

Step 3:
Molality is defined as moles of solute per kilogram of solvent. The number of moles is a count of particles, and kilograms is a measure of mass.

Step 4:
Unlike volume, the physical mass of a substance is a fundamental property that is absolutely unaffected by changes in temperature or pressure. A kilogram of water remains exactly a kilogram whether it is at $1^\circ$C or $99^\circ$C.

Step 5:
Because molality relies strictly on moles and mass, both of which are thermally stable, molality itself is completely independent of temperature. This makes it the preferred unit for thermodynamics calculations.