1. Given data

• Power of heater: $P = 60\,\text{W} = 60\,\text{J s}^{-1}$
• Time for which heater is on: $t = 100\,\text{s}$
• Temperature rise: $\Delta T = 5^\circ\text{C} = 5\,\text{K}$

2. Energy supplied by the heater

The electrical energy supplied is

$Q_{\text{supplied}} = Pt$

So,

$Q_{\text{supplied}} = 60 \times 100 = 6000\,\text{J}$

3. Thermodynamic condition

The gas is in a constant volume container with adiabatic walls.

• Adiabatic walls mean no heat is exchanged with surroundings.
• The heater is inside the gas, so the electrical energy supplied goes entirely into increasing the internal energy of the gas.
• At constant volume, no $PV$ work is done.

Hence,

$\Delta U = 6000\,\text{J}$

4. Heat capacity of the gas

For the given sample of gas,

$C = \frac{\Delta U}{\Delta T}$

Thus,

$C = \frac{6000}{5} = 1200\,\text{J K}^{-1}$

5. Final answer

$\boxed{1200}$