Question

Dissolving $120g$ of urea in $1000g$ of water gave a solution of density $1.15g/mL$. The molarity of the solution is:

• A. $1.79M$
• B. $2M$
• C. $2.05M$
• D. $2.22M$

Answer 1

Answer: (C)

$2.05M$

Dissolving $120g$ of urea in $1000g$ of water gave a solution of density $1.15g/mL$.

Molecular weight of urea is 60 g/mol.

The number of moles of urea $=\frac{\text{120 g}}{\text{60 g/mol}}=\text{2 mol}$

Total mass of solution $=\text{1000 g}+\text{120 g}=\text{1120 g}$

Total volume of solution $\frac{\text{1120 g}}{\text{1.15 g/mL}}=\text{973.9 mL}$

The molarity of the solution $=\frac{\text{2 mol}}{\text{973.9 mL}}\times \text{1000 mL}=\text{2.05 M}$

The molarity of the solution is $2.05M$.