Question

Dissolving $120\text{g}$ of urea (molecular weight $60\text{u}$) in $1000\text{g}$ of water gave a solution of density $1.15\text{g/mL}$. The molarity of the solution is:

• A. $1.78\text{M}$
• B. $3.00\text{M}$
• C. $2.05\text{M}$
• D. $4.22\text{M}$

Answer 1

The correct option is C $2.05\text{M}$

Number of moles of urea $=\frac{120}{60}=2$
Total mass of the solution $=1000+120=1120\text{g}$
$\text{Total volume of solution (in L)}=\frac{\text{mass}}{\text{demsity}}$
$=\frac{1120}{1.15\times {10}^3}=\frac{112}{115}\text{L}$
$\text{Molarity of the solution}=\frac{\text{Number of moles}}{\text{volume in L}}=\frac{2\times 115}{112}=2.02\text{M}$