According to equation, `C_(6)H_(6)(l)+15//2 O_(2)(g)rarr 6CO_(2)(g)+3H_(2)O(l), Delta H= -3264.4 "KJ mol"^(-1)` the energy evolved when 7.8 g benzene is burnt in air will be -

Options

A. 163.22 KJ
B. 32.64 KJ
C. 3.264 KJ
D. 326.4 KJ

Explanation

To calculate the energy evolved when 7.8 g of benzene (C₆H₆) is burnt in air, we can follow these steps:

### Step 1: Determine the molar mass of benzene (C₆H₆).
- The molar mass can be calculated as follows:
\[ \text{Molar mass of C₆H₆} = (12 \, \text{g/mol} \times 6) + (1 \, \text{g/mol} \times 6) = 72 \, \text{g/mol} + 6 \, \text{g/mol} = 78 \, \text{g/mol} \]

### Step 2: Calculate the number of moles of benzene in 7.8 g.
- Using the molar mass calculated in Step 1, we can find the number of moles:
\[ \text{Number of moles of C₆H₆} = \frac{\text{mass}}{\text{molar mass}} = \frac{7.8 \, \text{g}}{78 \, \text{g/mol}} = 0.1 \, \text{mol} \]

### Step 3: Use the enthalpy change (ΔH) to find the energy evolved.
- From the given reaction, we know that the combustion of 1 mole of benzene releases -3264.4 kJ of energy. Therefore, the energy released for 0.1 moles can be calculated as:
\[ \text{Energy evolved} = \Delta H \times \text{number of moles} = -3264.4 \, \text{kJ/mol} \times 0.1 \, \text{mol} = -326.44 \, \text{kJ} \]

### Step 4: Present the final answer.
- The energy evolved when 7.8 g of benzene is burnt in air is approximately:
\[ \text{Energy evolved} = -326.44 \, \text{kJ} \]
- Since we are interested in the magnitude, we can state:
\[ \text{Energy evolved} = 326.44 \, \text{kJ} \]

### Summary of the Solution:
The energy evolved when 7.8 g of benzene is burnt in air is approximately **326.44 kJ**.