Question

The molarity of urea (molar mass $60$ g $mo{l}^{-1}$) solution by dissolving $15$g of urea in $500$ $c{m}^3$ of water is?

• A. $2$ mol $d{m}^{-3}$
• B. $0.5$ mol $d{m}^{-3}$
• C. $0.125$ mol $d{m}^{-3}$
• D. $0.0005$ mol $d{m}^{-3}$

Answer 1

Answer: (B)

$0.5$ mol $d{m}^{-3}$

The number of moles of urea $=\frac{\text{Mass of urea}}{\text{Molar mass of urea}}=\frac{\text{15 g}}{\text{60 g/mol}}=\text{0.25 mol}$

The volume of solution is ${\text{500 cm}}^3\times \frac{\text{1 L}}{{\text{1000 cm}}^3}=\text{0.500 L}$

The molarity is the ratio of moles to the volume.

$\text{Molarity}=\frac{\text{Number of moles of solute}}{\text{Volume of solution (in L)}}$

$\text{Molarity}=\frac{\text{0.25 mol}}{\text{0.500 L}}$

$\text{Molarity}={\text{0.5 mol dm}}^{-3}$

So the answer is option $B$.