Related questions with answers
A solution of $2.50 \mathrm{~g}$ of a compound having the empirical formula $\mathrm{C}_6 \mathrm{H}_5 \mathrm{P}$ in $25.0 \mathrm{~g}$ of benzene is observed to freeze at $4.3^{\circ} \mathrm{C}$. Calculate the molar mass of the solute and its molecular formula.
Solutions
VerifiedThe problem asks to determine the molar mass of the unknown solute and to identify its molecular formula using the given empirical formula $\mathrm{C_6H_5P}$.
In this example, the required conversions are

Calculate the molality of the solution from the depression in freezing point

From molality, determine the number of moles.

Determine the mass.
We are going to use this approach:
$\begin{aligned} \Delta T_f = K_fm \tag1 \end{aligned}$
Where :

$\Delta T_f$ is the depression in the solution's freezing point

$K_f$ is the molal freezingpoint depression constant.

m is the molality.
To calculate a solution's molality, divide the number of moles of solute by the mass of solvent (in kg).
$\begin{aligned} \text{molality} = \dfrac{ \text{moles of solute}}{ \text{ mass of solvent } \tag2} \end{aligned}$
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