Question

A 0.727 g sample of D-ribose (C5H10O5) was placed in a bomb calorimeter (constant volume) and ignited in the presence of excess oxygen. The temperature was observed to rise by 0.910 K. In a separate experiment, 0.825 g of benzoic acid (C6H5CO2H) is similarly ignited in the same calorimeter, and is observed to cause an increase of the temperature of 1.940 K. The internal energy of combustion of benzoic acid is -3251 kJ mol-1. (a) Calculate the heat capacity of the calorimeter. (b) Calculate the internal energy of combustion of D-ribose. (c) Calculate the enthalpy of formation for D-ribose.

Answer 1

a) heat released during the combustion of benzoicacid (q) = n*DU

= (0.825/122.12)*-3251

   = -21.963 Kj

heat released during the combustion of benzoicacid (q) = C*DT

   c = heat capacity of calorimeter = ?

   DT = 1.94 k

21.963*10^3 = c*1.94

C = 11.321 kj/C

b) No of mol of D-ribose combusted = 0.727/150.13 = 0.00484 mol

   heat released(q) = c*DT

                    = 11.321*0.91

                    = 10.3 kj

   internal energy of combustion of D-ribose = q/n = -10.3/0.00484 = -2128.1 kj/mol

c)

2C5H10O5(s) + 10O2(g) ---> 10CO2(g) + 10H2O(l)

Dn = 10-10 = 0

so that, DU = DH

DH0combustion of D-ribose = -2128.1 kj/mol