Calculate the mass of urea `(NH_(2)CONH_(2))` required to prepare 2.5 kg of 0.25 molal aqueous solution.

Answer

37.5 g

Explanation

Molality of solution `= ("Mass of urea/Molar mass of urea")/("Mass of water in kg.")`
Molality of solution `=0.25 m=("0.25 mol kg"^(-1))`
Molar mass of urea `(NH_(2)CONH_(2))=2xx14+1xx12+1xx16+4xx1=60 "g mol"^(-1)`
Mass of solvent (water) = 2.5 kg
`("0.25 mol kg"^(-1))=("Mass of urea")/(("60 g mol"^(-1))xx("2.5 kg"))`
Mass of urea `=("0.25 mol kg^(-1))xx("60 g mol"^(-1))xx(2.5 kg)`
`= 38.5 g`.