The solution containing 3 g urea (molar mass | vedclass

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(D) The elevation in boiling point is given by $\Delta T_{b} = K_{b} \cdot m$. Since both solutions boil at the same temperature,their boiling point e

Vedclass · Accepted Answer

$90 \ g \ mol^{-1}$

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(D) The elevation in boiling point is given by $\Delta T_{b} = K_{b} \cdot m$. Since both solutions boil at the same temperature,their boiling point elevation $\Delta T_{b}$ must be equal. Assuming $K_{b}$ is the same for both dilute aqueous solutions,the molality $m$ must be equal. Molality $m = \frac{	ext{moles of solute}}{	ext{mass of solvent in } kg}$. For $1 \ dm^3$ of water,the mass is $1 \ kg$. For urea: $	ext{moles} = \frac{3 \ g}{60 \ g \ mol^{-1}} = 0.05 \ mol$. For solute $A$: $	ext{moles} = \frac{4.5 \ g}{M_A}$. Equating the molalities: $0.05 = \frac{4.5}{M_A}$. $M_A = \frac{4.5}{0.05} = 90 \ g \ mol^{-1}$.

The solution containing $3 \ g$ urea (molar mass $60 \ g \ mol^{-1}$) per $dm^3$ of water and another solution containing $4.5 \ g$ of solute $A$ per $dm^3$ of water boil at the same temperature. What is the molar mass of $A$?

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