Question

Concentration terms like mass percentage, ppm, mole fraction, and molality do not depend on temperature. However, molarity is a function of temperature because:
A. Volume depends on temperature and molarity involves volume.
B. Molarity involves non-volatile solute while the others involve volatile solutes.
C. Number of moles of the solute changes with change in temperature.
D. Molarity is used for polar solvents only.

Answer 1

Hint: Think about the definition of molarity, it is the number of moles of any substance per unit volume in liters. Compare this definition with the definitions of the other concentration terms.

Complete step by step answer:
Let us look at the formula that gives us the molarity of any solution:
\[\text{Molarity = }\dfrac{\text{no}\text{. of moles}}{\text{volume in liters}}\]
Here, we can see that the molarity depends on the number of moles of a substance present along with the volume of the given solution. One of the factors that affect volume is temperature due to the increase in kinetic energy and the bumping of molecules.
The mass percentage depends only on the masses of the components involved, be it solute and solution or atom and molecule. The formula is:
\[\text{mass }\!\!%\!\!\text{ = }\dfrac{\text{mass of solute}}{\text{mass of solution}}\times 100\]
The unit parts per million depends on the mass of any substance present in 1 liter of water. The formula is:
\[\text{ppm = }\dfrac{mg}{\text{liter}}\]
Mole fraction depends only on the number of moles of all the substances present in a solution. The number of moles depend on the given mass and the molecular mass of the substance. The formula is:
\[\text{mole fraction = }\dfrac{\text{no}\text{. of moles of solvent or solute}}{\text{no}\text{. of moles of solute + no}\text{. of moles of solvent}}\]
Molality is defined as the number of moles of a solute divided by the kilograms of a solvent present. Thus, this too, depends only on the masses of the substances. The formula for molality is:
\[\text{molality = }\dfrac{\text{no}\text{. of moles of solute}}{\text{mass of solvent in kgs}}\]
None of the formulae refer to the volatility, or the polar or nonpolar nature of the substances, it depends only on the volume and mass in case of molarity and mass incase of all the other formulae.
Thus, molarity is a function of temperature since it depends on the total volume of the solution.
Hence, the answer is ‘A. Volume depends on temperature and molarity involves volume.’

Note: Remember that although the formula for ppm includes liters, it is calculated as the number of particles found per liter of the solvent so the volume is constant. It is used for infinitesimally small concentrations and hence a small change in temperature will not affect the concentration in ppm.