Question

1) When 1.523 g of liquid pentane (C5H12) combusts in a bomb calorimeter, the temperature rises from 23.7?C to 29.8 ?C. What is ?Urxn for the reaction in kJ/mol pentane? The heat capacity of the bomb calorimeter is 5.23 kJ/?C.

2. The enthalpy of combustion of benzoic acid (C6H5CO2H) is -3228 kJ/mol. The burning of 1.698 g of benzoic acid in a calorimeter causes the temperature to increase by 2.865.C. What is the heat capacity in kJ/C) of the calorimeter?

Answer 1

Part A)

We have, Q = C x (Tf - Ti) = 5.23 x (29.8 - 23.7) = 31.903 KJ

Here, mass of pentane = 1.523 g

Molar mass of pentane = 72.146 g/mol

Moles of pentane = mass/molar mass = (1.523 )/(72.146) = 0.0211 moles

U_rxn = -Q/number of moles = -31.903/ 0.0211 = -1511.99 KJ/mol

U_rxn = -1511.99 KJ/mol

Part B)

Molar mass of benzoic acid = 122 g/mol

Mass of benzoic acid = 1.698 g

Moles of benzoic acid = mass / molar mass = 1.698/122 = 0.0139 mol

We have,

Heat released = IEnthalpy of combustionI x number of moles = 3228 x 0.0139 = 44.927 KJ

Now, we have

Q = C x (T_f - T_i)

44.927 KJ = C x 2.865 = 15.68 KJ/^oC

Heat capacity of the calorimeter = 15.68 KJ/^oC