The enthalpy of combustion of `C_(6)H_(6)(l)` is `-3250KJ` . When `039g` of benzene is burnt excess of oxygen in an open vessel, the amount of heat evolved is

Options

A. 16.25 J
B. 16.25 kJ
C. 32.5 J
D. 32.5 kJ

Answer

To solve the problem, we need to determine the amount of heat evolved when 0.39 g of benzene (C₆H₆) is burned in excess oxygen, given that the enthalpy of combustion of benzene is -3250 kJ per mole.

### Step-by-Step Solution:

1. **Write the balanced combustion reaction of benzene:**
\[ C_6H_6(l) + \frac{15}{2} O_2(g) \rightarrow 6 CO_2(g) + 3 H_2O(l) \]
This reaction shows that 1 mole of benzene combusts to produce carbon dioxide and water.

2. **Calculate the molar mass of benzene (C₆H₆):**
\[ \text{Molar mass of } C_6H_6 = (6 \times 12) + (6 \times 1) = 72 + 6 = 78 \text{ g/mol} \]

3. **Determine the number of moles of benzene in 0.39 g:**
\[ \text{Number of moles} = \frac{\text{mass}}{\text{molar mass}} = \frac{0.39 \text{ g}}{78 \text{ g/mol}} = 0.005 \text{ moles} \]

4. **Calculate the heat evolved for 0.39 g of benzene:**
Since the enthalpy of combustion of benzene is -3250 kJ per mole, the heat evolved for 0.005 moles can be calculated as follows:
\[ \text{Heat evolved} = \text{number of moles} \times \text{enthalpy of combustion} \]
\[ \text{Heat evolved} = 0.005 \text{ moles} \times (-3250 \text{ kJ/mol}) = -16.25 \text{ kJ} \]

5. **Final answer:**
The amount of heat evolved when 0.39 g of benzene is burnt in excess of oxygen is **16.25 kJ** (the negative sign indicates that heat is released).