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

Explanation

0.25 molal aqueous solution of urea means 0.25 moles of urea are present in 1000 g of water. Molar mass of urea `(NH_2 CONH_2) = 60 g "mol"^(-1)`
` therefore ` 0.25 mole of urea contains 0.25 mole `xx 60 g "mol"^(-1) = 15 g`
Total mass of solution = 1000 g + 15 g = 1015 g = 1.015 kg
Thus, 1.015 kg of solution contain urea = 15 g
2.5 kg of solution will require urea = `(15)/(1.015) xx 2.5 = 36.946 g `