Question

A solution containing $10$ g per dm${}^3$ of urea (m.w. = $60$) is isotonic with a $5\%$ solution of a non-volatile solute. The molecular mass of this non-volatile solute is:

• A. $250$ g mol${}^{-1}$
• B. $300$ g mol${}^{-1}$
• C. $350$ g mol${}^{-1}$
• D. $200$ g mol${}^{-1}$

Answer 1

The correct option is B $300$ g mol${}^{-1}$

${\pi }_1\left(urea\right)={\pi }_2\left(unknownsolute\right)$

$C_1\left(urea\right)=C_2\left(unknownsolute\right)$

Now,
${\left[\begin{array}{c}\frac{w_B\times 1000}{m_B\times V}\end{array}\right]}_{urea}={\left[\begin{array}{c}\frac{w_B\times 1000}{m_B\times V}\end{array}\right]}_{unknownsolute}$

$\frac{10\times 1000}{60\times 1000}=\frac{5\times 1000}{m_B\times 100}$

$m_B=300gmo{l}^{-1}$