Assertion : The solubility of salts of weak acids like phosphates decreases at lower pH.
Reason : The is because at lower pH concentration of cations increases.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion.
B. If both assertion and reason are true but reason is not the correct explanation of assertion.
C. If assertion is true but reason is false.
D. If both assertion and reason are false.

Explanation

To solve the question, we need to analyze the assertion and the reason provided. Here's a step-by-step breakdown:

### Step 1: Understand the Assertion
The assertion states: "The solubility of salts of weak acids like phosphates decreases at lower pH."

- **Explanation**: Salts of weak acids, such as phosphates, can dissociate in solution. When the pH of the solution decreases (meaning the concentration of H⁺ ions increases), it can affect the solubility of these salts.

### Step 2: Analyze the Effect of pH on Solubility
When the pH decreases, the concentration of H⁺ ions increases. For example, consider the dissociation of a phosphate salt:
\[ \text{HPO}_4^{2-} \rightleftharpoons \text{H}^+ + \text{PO}_4^{3-} \]

- **Le Chatelier's Principle**: According to Le Chatelier's principle, if we increase the concentration of one of the products (in this case, H⁺), the equilibrium will shift to the left to reduce the concentration of H⁺ ions. This means that the concentration of the phosphate ions (PO₄³⁻) will decrease.

### Step 3: Relate Ksp and Qsp
The solubility product constant (Ksp) for the dissociation of the salt can be expressed as:
\[ K_{sp} = [\text{Na}^+]^3 [\text{PO}_4^{3-}] \]

- **Effect on Ksp**: As the concentration of PO₄³⁻ decreases due to the shift in equilibrium, the Ksp value effectively decreases because it is dependent on the concentration of the anion.

### Step 4: Conclusion on Solubility
Since the concentration of PO₄³⁻ decreases, the Ksp decreases, which means that the solubility of the salt actually increases as the difference between Qsp and Ksp decreases.

### Step 5: Analyze the Reason
The reason states: "This is because at lower pH, the concentration of cations increases."

- **Explanation**: This statement is incorrect. The solubility change is primarily due to the decrease in the concentration of anions (PO₄³⁻), not an increase in cations. The assertion and reason do not support each other.

### Final Conclusion
Both the assertion and the reason are false:
- The assertion is false because the solubility of salts of weak acids like phosphates increases at lower pH.
- The reason is false because it incorrectly attributes the change in solubility to cation concentration rather than anion concentration.

### Answer
The correct option is D: Both assertion and reason are false.

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