POGIL Common Ion Effect on Solubility Extension Questions:
- Would the four beakers in Model 1 have the same reading on a pH meter? Explain your reasoning.
- Consider any of the beakers in Model 1. Predict what might happen to the solubility of the insoluble salt if a 1.0 M acid solution was added drop wise to the beaker. Hint: Which ion species in the beaker will react with the acid? How will this reaction alter the concentration of the common ion?
The Correct Answer and Explanation is :
16. Would the four beakers in Model 1 have the same reading on a pH meter? Explain your reasoning.
The four beakers in Model 1 likely would not have the same reading on a pH meter. The reason lies in the varying concentration of ions in each beaker and their effects on the pH. If each beaker contains a solution of a salt that dissociates into a common ion, the presence of that ion will influence the pH of the solution. For example, if one beaker contains a salt of a weak acid (like ammonium chloride, NH₄Cl), the pH may be slightly acidic because the ammonium ion (NH₄⁺) can release H⁺ ions into the solution. Conversely, if another beaker contains a salt of a weak base (like sodium acetate, NaOAc), the acetate ion (OAc⁻) could react with water to form OH⁻ ions, making the solution basic. Therefore, the common ion effect, the nature of the salt (acidic or basic), and the concentration of the salt will all impact the pH of the solution, making the pH readings in the four beakers different.
17. Consider any of the beakers in Model 1. Predict what might happen to the solubility of the insoluble salt if a 1.0 M acid solution was added dropwise to the beaker. Hint: Which ion species in the beaker will react with the acid? How will this reaction alter the concentration of the common ion?
When a 1.0 M acid solution is added dropwise to a beaker containing an insoluble salt, the solubility of the salt may increase. This is explained by the common ion effect and Le Chatelier’s principle. For example, if the salt is calcium carbonate (CaCO₃), it dissociates as follows:
[ \text{CaCO₃ (s)} \rightleftharpoons \text{Ca}^{2+} (aq) + \text{CO₃}^{2-} (aq) ]
When acid (H⁺) is added, it reacts with the carbonate ion (CO₃²⁻), forming carbonic acid (H₂CO₃), which quickly decomposes into water and CO₂ gas:
[ \text{CO₃}^{2-} + 2H⁺ \rightarrow \text{H₂CO₃} \rightarrow \text{H₂O} + \text{CO₂ (g)} ]
As the carbonate ions are removed from the solution due to this reaction, the system shifts to restore equilibrium by dissolving more solid salt to replace the carbonate ions. This causes the solubility of the salt to increase.
The presence of a common ion in the solution, such as the carbonate ion (CO₃²⁻) in the case of calcium carbonate, generally decreases the solubility of the salt because the equilibrium shifts to the left. However, by adding acid, the carbonate ions are consumed, and the equilibrium shifts to the right, leading to greater solubility.
Thus, adding a 1.0 M acid solution decreases the concentration of the carbonate ion, which reduces the common ion effect and increases the solubility of the insoluble salt in the solution.