Chemistry - Liquid Solution Question with Solution | TestHub

To calculate the molar mass of selenium (Se) $M_B=\frac{K_f\times W_B}{\Delta T_f\times W_A}$ Mas of selenium $\left(W_B\right)=3.26\mathrm{g}$ Mass of benzene $\left(W_A\right)=226\mathrm{g}=0.226\mathrm{kg}$ Depression in freezing point $\left(\Delta T_f\right)=0.112^{\circ }\mathrm{C}$ Molar depression constant $\left(K_f\right)=4.9\mathrm{K}\mathrm{kg}{\mathrm{mol}}^{-1}$ $\therefore M_B=\frac{\left(4.9\mathrm{K}\mathrm{kg}{\mathrm{mol}}^{-1}\right)(3.26\mathrm{g})}{(0.112\mathrm{K})(0.226\mathrm{kg})}=631.08\mathrm{g}{\mathrm{mol}}^{-1}$ Given, gram atomic mass of selenium $=78.8\mathrm{g}/\mathrm{mol}$ For ${\mathrm{Se}}_x\left(78.8\mathrm{g}{\mathrm{mol}}^{-1}\right)=631.08\mathrm{g}{\mathrm{mol}}^{-1}$ or, $x=\frac{631.08\mathrm{g}{\mathrm{mol}}^{-1}}{78.8\mathrm{g}{\mathrm{mol}}^{-1}}=8$ Thus, molecular formula of selenium $={\mathrm{Se}}_8$