Calculate the mass of urea \((NH_2CONH_2)\) required in making \(2.5 kg\) of \(0.25\) molal aqueous solution.

36.95 g

The correct answer is option 1. 36.95 g.

Molar mass of urea \((NH_2CONH_2)\) \(= 2 (1 × 14 + 2 × 1) + 1 × 12 + 1 × 16 = 60\, \ g\, \ mol^{−1}\)

\(0.25\) molar aqueous solution of urea means:

\(1000 g\) of water contains \(0.25\, \ mol\) \(= (0.25 × 60) \text{ g of urea = }15\text{ g of urea}\)

i.e, \((1000 + 15) g\) of solution contains \(15 g\) of urea

\(\frac{15×2500}{1000 + 15} g\)

Therefore, \(2.5 kg\) \((2500 g)\) of solution contains \(= \frac{15 × 2500}{1000 + 15} = 36.5\, \ g \text{ g of urea}\).

Hence, mass of urea required \(= 36.5 g\)