Question

260 g of an aqueous solution contains 60 g of urea (Molar mass = 60 g mol\(^{-1}\)). The molality of the solution is:

• A. 2m
• B. 3m
• C. 4m
• D. 5m
• E. 6m

Hint:

To calculate molality, use the formula \( m = \frac{{moles of solute}}{{mass of solvent in kg}} \).

Answer 1

The Correct Option is D

Step 1: Molality \( m \) is defined as the number of moles of solute per kilogram of solvent: \[ m = \frac{{moles of solute}}{{mass of solvent in kg}} \] Step 2: The number of moles of urea is: \[ {moles of urea} = \frac{{mass of urea}}{{molar mass}} = \frac{60 \, {g}}{60 \, {g/mol}} = 1 \, {mol} \] Step 3: The mass of solvent (water) is: \[ {mass of solvent} = 260 \, {g} - 60 \, {g} = 200 \, {g} = 0.2 \, {kg} \] Step 4: The molality is: \[ m = \frac{1 \, {mol}}{0.2 \, {kg}} = 5 \, {mol/kg} \] Thus, the molality of the solution is 5m.