By what property can the temperature difference be determined if the state of a substance does not change during the absorption of a given amount of heat?

Water of mass $5 \ kg$ in a closed vessel is at a temperature of $20^{\circ} C$. If the temperature of the water when heated for a time of $10 \ minutes$ becomes $30^{\circ} C$, then the increase in the internal energy of the water is (Specific heat capacity of water $= 4200 \ J \ kg^{-1} \ K^{-1}$) (in $kJ$)

How much heat energy is gained when $5\, kg$ of water at $20^{\circ}C$ is brought to its boiling point? (Specific heat of water $= 4.2\, kJ\, kg^{-1} {^{\circ}C}^{-1}$)

How much energy in $kJ$ is required to heat $5 \, kg$ of water from $20^oC$ to its boiling point? (Specific heat of water = $4.2 \, kJ \, kg^{-1} \, ^oC^{-1}$)

The temperature of $5$ moles of a gas at constant volume is changed from $100^{\circ} C$ to $120^{\circ} C$. The change in internal energy is $80 \,J$. The total heat capacity of the gas at constant volume in $J/K$ is: