What formula do I use if I want to calculate how much energy is absorbed by water?

Q = mc.dT

Q: heat absorbed
m: mass of water
c: specific heat capacity of water
dT: change in temperature

Can iuse the same formula to find the amount  per gram of fuel burned?

Q = mc.dT is applicable for physical processes. eg. transfer of heat to water.

Q = mc.dT is only applicable for burning fuel for this case:

1. burn 200g of fuel
2. heat evolved is used to heat water
3. all heat evolved from burning fuel channels to the water. [assumption]
4. water remains in liquid state.

heat evolved for burning 200g of fuel
= heat absorbed by water
= mc.dT where m: mass of water, c: specific heat capacity, dT: change in temperature.