Question

When $10.0$ ml of arsenious sulphide sol and $1.0\times {10}^{-4}MBaC{l}_2$ are mixed, turbidity due to precipitation appears after just $2$ hours. The effective ion and its coagulating value are, respectively?

• A. $C{l}^{-},$ $10$ mmol/L
• B. $C{l}^{-},$ $20$ mmol/L
• C. $B{a}^{2+},$ $10$ mmol/L
• D. $B{a}^{2+},$ $20$ mmol/L

Answer 1

The correct option is C $B{a}^{2+},$ $10$ mmol/L

The presence of small amounts of appropriate electrolytes is necessary for the stability of the colloids. However, when an electrolyte is added in larger concentration, the particles of the sol take up the ions which are oppositely charged and thus, get neutralised. The neutral particles then start aggregating giving particles of larger size which are then precipitated. This process of aggregation of colloidal particles into an insoluble precipitate by the addition of some suitable electrolyte is known as coagulation. It needs to be noted that the coagulation of a colloidal solution by an electrolyte does not take place until the added electrolyte has certain minimum concentration in the solution. The minimum concentration of electrolyte in millimoles required to cause coagulation of one litre of colloidal solution is called coagulation value.

Coagulation value = millimoles of electrolyte / volume of sol. in litre

Coagulating value $=\frac{{10}^{-4}\times {10}^3}{10\times {10}^{-3}}=10$ mmole/L.