Which of the following is correct?

Options

A. The pH of 1 L solution containing 0.49 g of `H_(2)SO_(4)` is 2.0
B. The conjugate base of `H_(2)S` is `S^(2-)`
C. `BF_(3)` is a Lewis base.
D. Phenolphthalein is colourless in basic medium.

Answer

To solve the question, we need to analyze the statements provided and determine which one is correct. Let's break down the solution step-by-step.

### Step 1: Analyze the first statement
The first statement claims that the pH of a 1-liter solution containing 0.49 grams of H2SO4 is 2.

1. **Calculate the number of moles of H2SO4:**
- Given mass = 0.49 grams
- Molecular mass of H2SO4 = (2 × 1) + (32) + (4 × 16) = 2 + 32 + 64 = 98 grams/mol
- Number of moles = Given mass / Molecular mass = 0.49 g / 98 g/mol = 0.005 moles

### Step 2: Calculate the concentration of H⁺ ions
2. **Calculate the concentration of H⁺ ions:**
- Since H2SO4 is a strong acid and dissociates completely, it produces 2 moles of H⁺ ions for every mole of H2SO4.
- Therefore, the concentration of H⁺ ions = 2 × (number of moles of H2SO4) = 2 × 0.005 moles = 0.01 moles/L

### Step 3: Calculate the pH
3. **Calculate the pH of the solution:**
- pH = -log[H⁺]
- pH = -log(0.01) = -log(10^-2) = 2

### Conclusion for the first statement
Thus, the pH of the solution is indeed 2, which confirms that the first statement is correct.

### Step 4: Analyze the other statements
4. **Examine the second statement:**
- The conjugate base of H2S is HS⁻, not S²⁻. Therefore, this statement is incorrect.

5. **Examine the third statement:**
- BF3 is a Lewis acid, not a Lewis base, as it accepts electron pairs. Hence, this statement is incorrect.

6. **Examine the fourth statement:**
- Phenolphthalein is colorless in acidic medium and turns pink in basic medium. Therefore, this statement is also incorrect.

### Final Answer
The only correct statement is the first one: **The pH of a 1-liter solution containing 0.49 grams of H2SO4 is 2.**

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