Question

$560$ g of $2$ m aqueous solution of urea is mixed with $2480$ g of $4$ m aqueous solution of urea. The molality of the resulting solution is :

• A. $3.6m$
• B. $4.0m$
• C. $4.6m$
• D. $4.9m$

Answer 1

The correct option is A $3.6m$

The molar mass of urea is 60 g/mol.

For 2 m solution, 1kg of water will contain 2 mol or 0.12kg urea.

The total mass of the solution will be 1.12 kg.

Hence, 560g of 2m solution will contain 0.06kg urea and 0.5kg water.

For, 4m solution, 1kg of water will contain 0.24kg urea. The total mass of the solution will be 1.24kg. Hence, 2480g of 4m solution will contain 0.48 kg urea and 2kg water.

Total mass of urea $=0.06kg+0.48kg=0.54kg$.

This is equal to 9moles. Total mass of water $=0.5kg+2kg=2.5kg$

The molarity of the solution is $m=\frac{9}{2.5}=3.6m$