0.5 g of benzoic acid was subjected to combustion in a bomb calorimeter at `15^(@)C` when the temperature of the calorimeter system (including water) was found to rise by `0.55^(@)C`. Calculate the heat of combustion of benzoic acid (i) at constant volume and (ii) at constant pressure. the thermal capacity of the calorimeter including water was found to be 23.85 kJ.

0.5 g of benzoic acid was subjected to combustion in a bomb calorimete

1.	0.5 g of benzoic acid was subjected to combustion in a bomb calorimeter at `15^(@)C` when the temperature of the calorimeter system (including water) was found to rise by `0.55^(@)C`. Calculate the heat of combustion of benzoic acid (i) at constant volume and (ii) at constant pressure. the thermal capacity of the calorimeter including water was found to be 23.85 kJ.
Answer» (i). Heat of combustion at constant volume, `DeltaU` =Heat capacity of calorimeter and its constent `xx`rise in temperature`xx("Mol. Mass of compound")/("Mass of compound")` `=23.85xx0.55xx(122)/(0.5)=3200.67kJ` i.e., `DeltaU=-3200.67kJ" "mol^(-1)` (ii) we know that, `DeltaH=DeltaU+DeltanRT` `C_(6)H_(5)COOH(s)+(15)/(2)O_(2)(g)to7CO_(2)(g)+3H_(2)O(l)` `Deltan=7-7.5=-0.5,R=8.314xx10^(-3)kJ" "K^(-1)mol^(-1)," "T=288K` Substituting the values in the above equation, `DeltaH=-3200.67-8.314xx10^(-3)xx(-0.5)xx288` `=-3200.67-1.197=-3201.867kJ" "mol^(-1)`

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