consider the following system at equilibrium.\ncaco₃(s) ⇌ ca²⁺(aq) + co₃²⁻(aq)\nthe addition of which…

Explanation:

Step1: Understand common - ion effect

The common - ion effect occurs when an ion in a chemical equilibrium is added from an external source, shifting the equilibrium according to Le - Chatelier's principle. In the given equilibrium $CaCO_3(s)\rightleftharpoons Ca^{2 + }(aq)+CO_3^{2 - }(aq)$, we need to find a compound that provides either $Ca^{2+}$ or $CO_3^{2 - }$ ions.

Step2: Analyze each option

• $CCl_4$: Does not dissociate into $Ca^{2+}$ or $CO_3^{2 - }$ ions in solution.
• $CO_2$: Does not directly provide $Ca^{2+}$ or $CO_3^{2 - }$ ions in a way that affects this equilibrium.
• $CuSO_4$: Dissociates into $Cu^{2+}$ and $SO_4^{2 - }$ ions, not relevant to the $Ca^{2+}$ or $CO_3^{2 - }$ ions in the given equilibrium.
• $Na_2CO_3$: Dissociates as $Na_2CO_3(s)\rightarrow 2Na^{+}(aq)+CO_3^{2 - }(aq)$. It provides the $CO_3^{2 - }$ ion which is a common ion in the $CaCO_3$ equilibrium.

Answer:

D. $Na_2CO_3$