Question

calculate the mass of Urea NH2 co NH2 required for making 2.5 kg of 0.25 molal solution​

Answer 1

Given

Mass of Solution = 2.5 kg.

molality of solution = 0.25 kg.

Formula

Molality of solution = $\frac{no \: of \: moles \: of \: solute}{weight \: of \: solvent in \: kg\: \: }$

Molar mass of urea = 60 gram

Solution

Let the weight of solute be x kg. Subsequently , weight of the solvent will be (2.5 - x ) kg.

moles of solute = $\frac{1000x}{60}$

Hence, molality =

$\frac{no \: of \: moles \: of \: solute}{weight \: of \: solvent \: in \: kg\: \: } \\ = > \frac{ \frac{1000x}{60} }{2.5 - x} = 0.25 \\ \\ = > \frac{1000x}{60} = 0.25(2.5 - x) \\ \\ = > \frac{1000x}{60} = 0.625 - 0.25x \\ \\ = > 1000x = 37.5 - 15x \\ \\ = > 1015x = 37. 5\\ \\ = > x = \frac{37.5}{1015} \\ \\ => x = 0.03 \: kg$

Hence, the mass of urea required is 0.03 kg.