A solution containing 10 g of urea (Mol. wt. 60) in one kg of water freezes at the same temperature as another solution containing 15g of solute S in same amount of water. Calculate molecular mass of S.

Explanation

To solve the problem, we will use the concept of molality and the fact that the freezing point depression (ΔTf) is a colligative property, which depends on the number of solute particles in solution rather than their identity.

### Step-by-Step Solution:

1. **Understand the Concept of Molality**:
Molality (m) is defined as the number of moles of solute per kilogram of solvent. It is given by the formula:
\[
m = \frac{\text{moles of solute}}{\text{mass of solvent in kg}}
...