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Q.
A $42.12 \% ( w / v )$ solution of $NaCl$ causes precipitation of a certain sol in 10 hours. The coagulating value of $NaCl$ for the sol is
[Given : Molar mass : $Na =23.0\,g\, mol ^{-1} ; Cl =35.5$ $\left.g mol ^{-1}\right]$
Here $42.12 \%(w / v)$
$\Rightarrow 42.12$ gram salt in $100 ml$ water
$\Rightarrow 421.2$ gram salt $( NaCl )$ in $1000 ml$ or 1 lit water
$\Rightarrow \frac{421.2}{58.5}=7.2$ moles of salt
$7.2$ moles $\rightarrow 10$ hours
$x$ moles $\rightarrow 2$ hours
$
\begin{array}{l}
\Rightarrow x=\frac{7.2 \times 2}{10}=1.44 mol / L \\
\Rightarrow x=1440 mmol / L
\end{array}
$