Which of the following compounds, when added to a saturated solution of AgCl, will cause a shift in equilibrium due to the common ion effect?

• A. AgNO3
• B. Na2SO4
• C. NaCl
• D. KNO3

Answer: (C)

Verified step by step guidance

1. Recall the dissolution equilibrium of silver chloride (AgCl): \(\mathrm{AgCl (s) \rightleftharpoons Ag^{+} (aq) + Cl^{-} (aq)}\). The system is at equilibrium when the product of the ion concentrations equals the solubility product constant, \(K_{sp}\).
2. Understand the common ion effect: adding a compound that provides an ion already present in the equilibrium will shift the equilibrium position according to Le Chatelier's principle, usually decreasing the solubility of the salt.
3. Identify the ions involved in the AgCl equilibrium: \(\mathrm{Ag^{+}}\) and \(\mathrm{Cl^{-}}\). Any compound that supplies either \(\mathrm{Ag^{+}}\) or \(\mathrm{Cl^{-}}\) ions will affect the equilibrium.
4. Examine each compound: AgNO\(_3\) provides \(\mathrm{Ag^{+}}\) ions, NaCl provides \(\mathrm{Cl^{-}}\) ions, Na\(_2\)SO\(_4\) provides \(\mathrm{SO_4^{2-}}\) ions (not involved in the equilibrium), and KNO\(_3\) provides \(\mathrm{K^{+}}\) and \(\mathrm{NO_3^{-}}\) ions (also not involved).
5. Conclude that only AgNO\(_3\) and NaCl will cause a shift in the AgCl equilibrium due to the common ion effect, because they supply ions common to the dissolution equilibrium.