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  4. NaCl is added to 1 litre water to such an extent that Δ Tf/Kf becomes to (1/500), the wt. of NaCl added is

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Q. NaCl is added to 1 litre water to such an extent that $\Delta\,T_{f}/K_{f}$ becomes to $\frac{1}{500}$, the wt. of NaCl added is

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Solution:

For $NaCl, i = 2$
$\therefore \frac{\Delta T_{f}}{K_{f}}=2\times C_{m}$
$\therefore \Delta T_{f}=K_{f} \times i\times C_{m}$
or, $\frac{1}{500}=2\times$ molality
or molality $=\frac{1}{1000}$
i.e., $\frac{1}{1000}$ moles $kg^{-1}$ water
i.e., $\frac{1}{1000}$ moles per litre water
$=\frac{1}{1000}\times58.5\,g\, Nacl$
$=0.0585\,g\,Nacl$