Question

A saturated \(CaCO_3 \)stock solution is existing at 25°C. In one experiment (i) 25 g \(Na_2CO_3 \)is added to the stock solution. In another experiment (ii) 25 g \(Na_2SO_4\) is added to the stock solution. Select the correct statement from the following.

• A. Addition of (i) increases the concentration of \( Ca^{2+}\) and addition of (ii) decreases the concentration of \(Ca^{2+}\)
• B. Addition of (i) decreases the concentration of \(Ca^{2+}\) and addition of (ii) increases the concentration of \(Ca^{2+}\)
• C. Addition of (i) and (ii) increase the concentration of \(Ca^{2+}\)
• D. Addition of (i) and (ii) decrease the concentration of \(Ca^{2+}\)

Answer 1

The Correct Option is B

The addition of Na$_2$CO$_3$ increases the carbonate ion concentration in the solution, which via the common ion effect, decreases the solubility of CaCO$_3$, thus reducing the concentration of Ca$^{2+}$. Contrarily, the addition of Na$_2$SO$_4$ increases the ionic strength of the solution which could enhance the solubility of CaCO$_3$ through the salt effect, paradoxically increasing Ca$^{2+}$ concentration despite potential precipitation of CaSO$_4$.