Name: Condition for Precipitation
Uploaded: 2019-07-19
Duration: 6 min 35 s
Description: The video is based on the condition for precipitation of a salt dissolved in a solvent, where the ionic product should exceed the solubility product of the s...

Question

Explain solubility product and common ion effect? Discuss their applications in qualitative analysis.

Accepted Answer

Solubility product:

The solubility product constant is the equilibrium constant for the dissolution of a solid substance into an aqueous solution. It is denoted by the symbol $K_{sp}$ .

The solubility product is a kind of equilibrium constant and its value depends on temperature.

Common ion effect:

The degree of dissociation of an electrolyte or weak acid or weak base is suppressed by the addition of another electrolyte containing a common ion. This phenomenon is termed as common ion effect, e.g. is a weak acid and ionises in the following manner.

$HAC (aq) ⇌ H^{+} (aq) + {AC}^{-} (aq); K_{a} = \frac{[H^{+}] [{AC}^{-}]}{[HAC]}$

This is common ion effect and may be defined as the suppression of the degree of dissociation of a weak acid or a weak base by the addition of a strong electrolyte containing a common ion.

Qualitative analysis

The common ion effect is generally employed in qualitative analysis.

The cations of group $II$ $({Hg}^{2 +}, {Pb}^{2 +}, {Bi}^{3 +}, {Cu}^{2 +}, {As}^{3 +}, {Sb}^{3 +}, {Sn}^{2 +})$ are precipitated as their sulphides (such as $CuS, PbS$ ) by passing $H_{2} S$ gas in the presence hydrochloric acid (Common $H^{+}$ ions).

The cations of group $III$ are precipitated as their hydroxides by ${NH}_{4} OH$ in the presence of ${NH}_{4} Cl$ .

The cations of group $V$ are precipitated as their carbonates by the addition ${({NH}_{4})}_{2} {CO}_{3}$ , in the presence of

Purification of sodium chloride Sodium chloride obtained from sea-water or lakes is always impure. It can be purified on the basis of common ion effect as described below:

The saturated solution of impure sodium chloride is prepared by dissolving in minimum quantity of water. $HCl$ gas is then passed through this solution.

Explain solubility product and common ion effect? Discuss their applications in qualitative analysis.

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