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

Explanation:

Step1: Understand common - ion effect

The common - ion effect occurs when an ion in common with one of the ions in the equilibrium is added to the system. In the given equilibrium $CaCO_3(s)\rightleftharpoons Ca^{2 + }(aq)+CO_3^{2 - }(aq)$, we need to find a compound that will introduce either $Ca^{2+}$ or $CO_3^{2 - }$.

Step2: Analyze each option

• $CCl_4$: Does not contain $Ca^{2+}$ or $CO_3^{2 - }$ ions. It is a non - electrolyte in this context and will not cause a shift due to the common - ion effect.
• $CO_2$: Does not contain $Ca^{2+}$ or $CO_3^{2 - }$ ions relevant to this equilibrium in a way that would cause a common - ion effect.
• $CuSO_4$: Contains $Cu^{2+}$ and $SO_4^{2 - }$ ions, not $Ca^{2+}$ or $CO_3^{2 - }$ ions, so it will not cause a shift due to the common - ion effect.
• $Na_2CO_3$: Dissociates in solution as $Na_2CO_3(s)\rightarrow 2Na^{+}(aq)+CO_3^{2 - }(aq)$. It introduces the $CO_3^{2 - }$ ion which is common to the equilibrium $CaCO_3(s)\rightleftharpoons Ca^{2 + }(aq)+CO_3^{2 - }(aq)$. According to Le Chatelier's principle, the addition of $CO_3^{2 - }$ will shift the equilibrium to the left to reduce the excess of $CO_3^{2 - }$ ions.

Answer:

D. $Na_2CO_3$