The mass of urea \((NH_2CONH_2)\) required to make \(2.5\, \ kg\) of \(0.25\) molal aqueous solution will be:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 36.95 g
Molality is the number of moles of solute per kilogram of solvent. Here 2.5 kg is the mass of the whole solution, so the urea in it must be counted out to get the mass of water.
Molar mass of urea, \( \ce{NH2CONH2} \) = 2(14) + 4(1) + 12 + 16 = 60 g mol−1
A 0.25 molal solution has 0.25 mol of urea = 0.25 × 60 = 15 g of urea in 1000 g of water, that is, in 1015 g of solution.
Mass of urea in 2500 g of solution:
\( \displaystyle \frac{15\ \text{g} \times 2500\ \text{g}}{1015\ \text{g}} = 36.95\ \text{g} \)
Note: This question is from NCERT (2026-27). NTA sets numerical questions from NCERT too, so do not ignore NCERT while practising numericals.
NCERT (2026-27 edition):
Intext Questions – “1.4 Calculate the mass of urea (NH2CONH2) required in making 2.5 kg of 0.25 molal aqueous solution.”