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 determine which of the statements provided is correct, we will analyze each statement step by step.

### Step 1: Calculate the pH of a 1-liter solution containing 0.49 g of H₂SO₄.

1. **Calculate the number of moles of H₂SO₄**:
- The molar mass of H₂SO₄ (sulfuric acid) is calculated as follows:
- H: 1 g/mol × 2 = 2 g/mol
- S: 32 g/mol
- O: 16 g/mol × 4 = 64 g/mol
- Total = 2 + 32 + 64 = 98 g/mol
- Now, calculate the number of moles in 0.49 g:
\[ \text{Moles of H₂SO₄} = \frac{0.49 \text{ g}}{98 \text{ g/mol}} = 0.005 \text{ moles} \]

2. **Determine the concentration of H₂SO₄**:
- Since the solution volume is 1 liter, the concentration of H₂SO₄ is:
\[ \text{Concentration} = 0.005 \text{ moles/L} = 5 \times 10^{-3} \text{ M} \]

3. **Calculate the concentration of H⁺ ions**:
- H₂SO₄ is a strong acid and dissociates completely:
\[ \text{H₂SO₄} \rightarrow 2 \text{H}^+ + \text{SO₄}^{2-} \]
- Therefore, the concentration of H⁺ ions will be:
\[ [\text{H}^+] = 2 \times 5 \times 10^{-3} = 1 \times 10^{-2} \text{ M} \]

4. **Calculate the pH**:
- The pH is calculated using the formula:
\[ \text{pH} = -\log[\text{H}^+] \]
- Substituting the concentration of H⁺:
\[ \text{pH} = -\log(1 \times 10^{-2}) = 2 \]

### Conclusion for Step 1:
The first statement is correct: "The pH of a 1-liter solution containing 0.49 g of H₂SO₄ is 2."

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### Step 2: Analyze the second statement: "The conjugate base of H₂S is H₂-."

1. **Identify the conjugate base**:
- The conjugate base of an acid is formed by removing a proton (H⁺) from the acid.
- For H₂S, removing one H⁺ gives us HS⁻, not H₂-.

### Conclusion for Step 2:
The second statement is incorrect.

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### Step 3: Analyze the third statement: "BF₃ is a Lewis base."

1. **Identify the nature of BF₃**:
- BF₃ (Boron trifluoride) has an incomplete octet and can accept electrons, making it a Lewis acid, not a base.

### Conclusion for Step 3:
The third statement is incorrect.

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### Step 4: Analyze the fourth statement: "Phenolphthalein is colorless in basic medium."

1. **Understand the behavior of phenolphthalein**:
- Phenolphthalein is colorless in acidic solutions (pH < 8) and turns pink in basic solutions (pH > 10).

### Conclusion for Step 4:
The fourth statement is incorrect.

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### Final Conclusion:
The only correct statement is the first one: "The pH of a 1-liter solution containing 0.49 g of H₂SO₄ is 2."

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