Question

What is the mass of urea required for making $2.5kg$ of $0.25$ molal aqueous solution?

• A. $37g$
• B. $25g$
• C. $125g$
• D. $27.5g$

Answer 1

Answer: (A)

No. of moles of urea $=0.25$ moles
Mass of solvent $=1kg=1000g$
Molar mass of urea $=60gmo{l}^{-1}$
Mass of urea in $1000g$ of $H_{2}O=0.25\times 60=15g$
Total mass of solution $=1000+15=1015g$
$1.015kg$ solution contains $15g$ urea
$2.5kg$ solution will contain $=\frac{15}{1.015}\times 2.5=37g$