(a) When a 0.235-g sample of benzoic acid is combusted in a
bomb calorimeter, the temperature rises 1.642 °C.
When a 0.265-g sample of caffeine, C8H10O2N4, is burned, the
temperature rises 1.525 °C. Using the value for the 26.38 kJ/g
heat of combustion of benzoic acid, calculate the heat of combustion
per mole of caffeine at constant volume.

The first target is grams of water. So, 26.38 kJ/g * .235 g of benzoic acid = 6.1993 kJ or 6199.3 J of heat (Q). Now the formula is Q= m dT Cp (heat = mass(water) *change in temp*specific heat of water (4.186 J/gC)). 6199.3=m*1.642C*4.186J/gC so mass of water is 901.92 g. Now you have the mass of the water to plug into the second part. Q = 901.92*1.525*4.186 = 5757.57 or 5.757 kJ. Divide this by .265 g to get kJ/g to be roughly 21.73 kJ/g.