Question

Mohr's method of precipitation titration relies on the fact that silver reacts preferentially with chloride ions instead of chromate ions in solution. When I searched up why this is true, the only response was that the Ksp of AgCl (1.77×10−10) is greater than the Ksp of Ag2CrO4 (1.2×10−12):

However this doesn't make sense to me since shouldn't the more soluble a compound is indicate that it is less likely to react and form a precipitate meaning that by this logic shouldn't Ag2CrO4 be preferential not AgCl? Could someone explain what is wrong with this reasoning and how to actually explain why silver reacts preferentially with chloride ions?

Answer 1

The propensity for silver chloride to precipitate before silver chromate in Mohr's method is due to its higher solubility product (1.77×10⁻¹⁰ for AgCl vs. 1.2×10⁻¹² for Ag₂CrO₄), indicating silver chloride precipitates at a lower Ag+ concentration.

Step-by-step explanation:

The confusion regarding Mohr's method of precipitation titration and the selective precipitation of silver salts like AgCl and Ag2CrO4 lies in the interpretation of their solubility products (Ksp). A smaller Ksp value does not indicate higher solubility but rather lower solubility, meaning a compound is less likely to dissolve in solution and more likely to form precipitate.

The precipitation of silver halides (such as AgCl and AgBr) in equilibria can be influenced by the initial concentrations of the halide ions in solution. For instance, if AgNO3 is gradually added to a solution containing more Cl- ions than Br- ions, AgCl will precipitate first due to its lower solubility in comparison to AgBr, despite AgBr having a smaller Ksp.