Question

Which of the following concentration terms depends on temperature?

• A. Molality
• B. Molarity
• C. Mole fraction
• D. Percent by mass

Hint:

Logic Tip: A golden rule in physical chemistry: Any concentration unit with a "V" (Volume) in its formula changes with temperature. Any unit with only "W" or "n" (Weight or Moles) is temperature independent.

Answer 1

The Correct Option is B

Concept:
Concentration terms can be categorized based on whether they involve the volume of the solution or only the mass/moles of the components. Volume expands or contracts with changes in temperature (thermal expansion), whereas mass and moles remain strictly constant regardless of temperature.
Step 1: Analyze the formulas of the given concentration terms.

• Molality (m): Moles of solute / Mass of solvent (in kg). Involves only moles and mass. (Independent of Temperature)
• Mole fraction ($\chi$): Moles of component / Total moles in mixture. Involves only moles. (Independent of Temperature)
• Percent by mass (% w/w): Mass of solute / Total mass of solution $\times$ 100. Involves only mass. (Independent of Temperature)
• Molarity (M): Moles of solute / Volume of solution (in L). Involves volume.

Step 2: Identify the temperature-dependent term.
Since Molarity depends on the volume of the solution, and volume changes with temperature, the molarity of a solution will change as the temperature changes. As temperature increases, volume generally increases, causing Molarity to decrease.