When excess oxalic acid si added to `CaCI_(2), CaC_(2)O_(4)` is precipitated but the solution still contains some unprecipated `Ca^(2+)` because

Options

A. `CaC_(2)O_(4)` soluble
B. oxalic acid does not ionise
C. when `H_(2)C_(2)O_(4)` is added to `CaI_(2)` HCI, is formed which is fully ionised and thus increases the `H^(+)` concentration suppresses the ionisation of `H_(2)C_(2)O_(4)` and hense the solubility product of `CaC_(2)O_(4)` is not exceeded
D. None of the above

Answer

To solve the question regarding the precipitation of calcium oxalate (CaC2O4) when excess oxalic acid (H2C2O4) is added to calcium chloride (CaCl2), we can follow these steps:

### Step 1: Understand the Reaction
When calcium chloride (CaCl2) is mixed with oxalic acid (H2C2O4), a precipitation reaction occurs where calcium oxalate (CaC2O4) is formed. The reaction can be represented as:
\[ \text{CaCl}_2 + \text{H}_2\text{C}_2\text{O}_4 \rightarrow \text{CaC}_2\text{O}_4 \downarrow + 2 \text{HCl} \]

### Step 2: Ionization of HCl
The HCl formed in the reaction is a strong acid and fully ionizes in solution:
\[ \text{HCl} \rightarrow \text{H}^+ + \text{Cl}^- \]

This increases the concentration of hydrogen ions (H⁺) in the solution.

### Step 3: Dissociation of Oxalic Acid
Oxalic acid (H2C2O4) can dissociate in the following steps:
1. First dissociation:
\[ \text{H}_2\text{C}_2\text{O}_4 \rightleftharpoons \text{H}^+ + \text{HC}_2\text{O}_4^- \]
2. Second dissociation:
\[ \text{HC}_2\text{O}_4^- \rightleftharpoons \text{H}^+ + \text{C}_2\text{O}_4^{2-} \]

### Step 4: Effect of Increased H⁺ Concentration
The increase in H⁺ concentration due to the ionization of HCl shifts the equilibrium of the dissociation of oxalic acid to the left (Le Chatelier's Principle). This means that the concentration of oxalate ions (C2O4²⁻) decreases.

### Step 5: Ionic Product vs. Solubility Product
For precipitation to occur, the ionic product (IP) must be greater than the solubility product (Ksp) of calcium oxalate:
\[ \text{IP} = [\text{Ca}^{2+}][\text{C}_2\text{O}_4^{2-}] \]
However, due to the decrease in the concentration of C2O4²⁻ ions, the ionic product may fall below the solubility product, preventing further precipitation of CaC2O4.

### Conclusion
As a result, even though excess oxalic acid is added, some calcium ions (Ca²⁺) remain unprecipitated in the solution because the increased concentration of H⁺ ions suppresses the formation of oxalate ions, thus reducing the ionic product below the solubility product of calcium oxalate.

### Final Answer
The solution still contains some unprecipitated Ca²⁺ because the increase in H⁺ concentration from the ionization of HCl suppresses the ionization of oxalic acid, leading to a decrease in the concentration of C2O4²⁻ ions, which in turn reduces the ionic product below the solubility product of CaC2O4.

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