question_answer 4)

Calculate the mass of urea (NH2CONH2) required in making 2.5 kg of 0.25 molal aqueous solution.

Answer:

0.25 molal aqueous solution of urea means that 0.25 mole of urea are present in 1 kg of water. Moles of urea = 0.25 mole Mass of solvent (water) = 1 kg = 1000 g Molar mass of urea (NH2CONH2) = 60 g mol?1. 0.25 mole of urea = 0.25 mot x 60 g mol?1 = 15 g Total mass of solution = 1000 + 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