Question

Two unknown molecular compounds were being studied. A solution containing 5.00 g of compound A in 100. g of water froze at a lower temperature than a solution containing 5.00 g of compound B in 100. g of water. Which compound has the greater molar mass? Explain how you arrived at your answer.

Answer 1

Answer:

Compound B.

Explanation:

The freezing point depression is a colligative property. It depends on the number of particles (moles) present in the solution.

$\Delta T_{\text{f}} = K_{\text{f}} b$

where b is the molal concentration

$b = \dfrac{\text{moles of solute}}{\text{kilograms of solvent}}\\\\n = \dfrac{\text{mass}}{\text{molar mass}} = \dfrac{m}{M}$

If m is constant (5 g), then

$n \propto \dfrac{1}{M}$

The compound with the greater molar mass has fewer moles and therefore fewer particles to depress the freezing point.

That must be Compound B, because Compound A has the lower freezing point.