Question

Which of the following will precipitate first when an aqueous solution containing sulphate ions is added?

• A. \( {Mg}^{2+} \)
• B. \( {Ca}^{2+} \)
• C. \( {Sr}^{2+} \)
• D. \( {Ba}^{2+} \)

Hint:

As we go down Group 2, the solubility of sulphates decreases. Barium sulphate (\({BaSO}_4\)) is the least soluble and precipitates first when sulphate ions are added.

Answer 1

The Correct Option is D

Step 1: Understanding solubility trends. The precipitation of metal sulphates depends on their solubility product constant (\( K_{sp} \)). The lower the \( K_{sp} \) value, the less soluble the compound is, meaning it will precipitate first. 
Step 2: Solubility of sulphates. The solubility order of alkaline earth metal sulphates (\({M}^{2+} {SO}_4^{2-}\)) decreases down the group: \[ {MgSO}_4>{CaSO}_4>{SrSO}_4>{BaSO}_4 \] Since barium sulphate (\({BaSO}_4\)) has the lowest solubility, it will precipitate first.