The minimum concentration of an electrolyte in millimoles per litre required to cause precipitation of a sol in two hours is called coagulating value.

$$\begin{gathered} 42.12 \% \mathrm{means} 100 \mathrm{ml} \mathrm{solution} \mathrm{contain} 42.12 \mathrm{g} \mathrm{of} \mathrm{salt}. \\ \mathrm{moles} \mathrm{of} \mathrm{salt} = \frac{42.12}{58.5} = 0.72 \mathrm{moles} = 720 \mathrm{milimoles} \end{gathered}$$

Since 100 ml required 720 milimoles

therefor 1 L will required 7200 milimoles.

It will coagulate in 10 hours so for 2 hours = $5\times 7200 = 14400 \mathrm{mili} \mathrm{mol} {\mathrm{L}}^{-1}$

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