Which one of the following statements is not true ?

Options

A. The conjugate base of `H_2PO_4^(-) " is " HPO_4^(2-) `
B. pH+ pOH = 14 for all aqueous solutions
C. The pH of ` 1 xx 10^(-8) M ` HCl is 8
D. `96,500` coulombs of electricity when passed through a `CuSO_4` solution deposite 1g equivalent of copper at the cathode.

Explanation

To determine which statement is not true, we will analyze each statement provided in the question step by step.

### Step 1: Analyze Statement A
**Statement A:** The conjugate base of H₂PO₄⁻ is HPO₄²⁻.

- **Explanation:** The conjugate base of an acid is formed when the acid donates a proton (H⁺). H₂PO₄⁻ can lose one hydrogen ion to form HPO₄²⁻. Thus, this statement is true.

### Step 2: Analyze Statement B
**Statement B:** pH + pOH = 40.

- **Explanation:** The relationship between pH and pOH is given by the equation pH + pOH = pK_w at 25°C, where pK_w is 14. Therefore, the statement that pH + pOH = 40 is incorrect at standard conditions. This statement is false.

### Step 3: Analyze Statement C
**Statement C:** The pH of 10⁻⁸ M HCl is 8.

- **Explanation:** For a 10⁻⁸ M HCl solution, HCl is a strong acid and will completely dissociate. Therefore, the concentration of H⁺ ions from HCl will be 10⁻⁸ M. Additionally, the water contributes H⁺ ions at a concentration of 10⁻⁷ M. The total concentration of H⁺ ions will be 10⁻⁸ + 10⁻⁷ = 1.1 × 10⁻⁷ M. The pH is calculated as:
\[ \text{pH} = -\log(1.1 \times 10^{-7}) \approx 7 \]
Therefore, this statement is also false.

### Step 4: Analyze Statement D
**Statement D:** According to Faraday's law of electrolysis, W = E × Q / 96500.

- **Explanation:** Faraday's law states that the weight (W) of the substance deposited is proportional to the charge (Q) passed through the electrolyte and is given by the formula:
\[ W = \frac{E \times Q}{F} \]
where F is Faraday's constant (approximately 96500 C/mol). This statement is true.

### Conclusion
From the analysis, we find that both statements B and C are false. However, since the question asks for one statement that is not true, we can conclude that:

**The answer is Statement B: pH + pOH = 40.**