1 mole of benzene=78 gm. As we know heat of combustion is only for1 mole. In 78gm bengene, delta H =-326.4 kj . And for 7.8gm ,delta H= -326.44kj

Benzene is a chemical compound with the formula C6H6. When benzene is burned with oxygen, it undergoes a combustion reaction to produce water (H2O) and carbon dioxide (CO2). The enthalpy change (∆H) for this reaction is -3264.4 kJ.

To calculate the energy evolved when 7.8g of benzene is burnt in air, we can use the following steps:

1. Calculate the moles of benzene:
- The molar mass of benzene (C6H6) is the sum of the molar masses of carbon (C) and hydrogen (H) atoms.
- Carbon has a molar mass of 12.01 g/mol, and hydrogen has a molar mass of 1.01 g/mol.
- The molar mass of benzene is therefore (6 * 12.01) + (6 * 1.01) = 78.11 g/mol.
- Divide the given mass of benzene (7.8g) by its molar mass to find the number of moles: 7.8g / 78.11 g/mol = 0.1 mol.

2. Calculate the energy evolved:
- The enthalpy change (∆H) for the combustion of benzene is -3264.4 kJ.
- Multiply the moles of benzene by the ∆H value to find the energy evolved: 0.1 mol * -3264.4 kJ/mol = -326.44 kJ.

3. Determine the sign of the energy evolved:
- The negative sign indicates that the reaction is exothermic, meaning that energy is released during the combustion of benzene.

Therefore, the energy evolved when 7.8g of benzene is burnt in air is -326.44 kJ. This means that the combustion reaction releases 326.44 kJ of energy.

In summary:
- Mass of benzene: 7.8g
- Molar mass of benzene: 78.11 g/mol
- Moles of benzene: 0.1 mol
- ∆H for combustion of benzene: -3264.4 kJ
- Energy evolved: -326.44 kJ (exothermic reaction)