Question

what is the principle behind adding excuess BaCl2 to ensure complete precipitation of the SO4^-2?
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Answer 1

Answer on Question #55913 - Chemistry - General chemistry

Question:

What is the principle behind adding excess $\mathrm{BaCl}_2$ to ensure complete precipitation of the $\mathrm{SO}_4^{2-}$?

Answer:

$\mathrm{BaSO_4}$ is not totally insoluble in water. Due to chemical dissociation process, there's still some amount of dissolved anions and cations is presented in the solution. The chemical equilibrium is described below:

$$\mathrm{BaSO_4}(s) \rightleftharpoons \mathrm{Ba^{2+}}(aq) + \mathrm{SO_4^{2-}}$$

The chemical equilibrium is almost shifted to the left, and the major part of $\mathrm{BaSO_4}$ is in the solid state. Anyway, the equilibrium exists and we can write a solubility product constant, which is written $K_{sp}$:

$$K_{sp} = \left[ \mathrm{Ba^{2+}}(aq) \right] \left[ \mathrm{SO_4^{2-}}(aq) \right]$$

By adding excess of $\mathrm{Ba^{2+}}$ we ensure that the chemical equilibrium is shifted to the left side, so all $\mathrm{SO_4^{2-}}$ anions are in the solid state, that means that they don't remain in the solution.

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