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# The expression relating molality (m) and mole fraction of solute $(x_2)$ in a solution is (where $M_1$ is the molar mass of solvent)

$\begin{array}{1 1}x_2=\large\frac{mM_1}{1+mM_1}\\x_2=\large\frac{mM_1}{1-mM_1}\\x_2=\large\frac{1+mM_1}{mM_1}\\x_2=\large\frac{1-mM_1}{mM_1}\end{array}$

Answer : $x_2=\large\frac{mM_1}{1+mM_1}$
We have
$x_2=\large\frac{n_2}{n_1+n_2}=\frac{n_2}{(m_1/M_1)+n_2}=\frac{n_2M_1}{m_1+n_2M_1}=\frac{(n_2/m_1)M_1}{1+(n_2/m_1)M_1}=\frac{mM_1}{1+mM_1}$