Heat of combustion of glucose at constant pressure at 17°C was found to be -651,000 cal. Calculate the heat of combustion of glucose at constant volume considering water to be in the gaseous state.

C6H12O6(s)+6O2(g)6CO2(g)+6H2O(g)   negligiible 6mole6mole6mole Volume Δn=12−6=6ΔE=ΔH−ΔnRT=−651000−6×2×(273+17)=−651000−3480=−654480calHence, heat of combustion of glucose at constant volume and atl7°C is -654,480 cal.