Question

Benzoic acid, C6H5COOH, is a common standard used in
bomb calorimeters, which maintain a constant volume. If 1.20 g
of benzoic acid gives off 31,723 J of energy when burned in
the presence of excess oxygen and in a water bath having
a temperature of 24.6°C, calculate q, w, DH, and DU for the
reaction.

Answer 1

Bnezoic acid moles = mass / molar mass of benzoic acid = 1.2 / 122.2 = 0.00982

Heat given = 31723 Joules per 0.00982 moles Benzoic acid

Heat given per mole = 31723 /0.00982 = 3230448 J = 3230.5 KJ

Thus Q   = heat   given = - 31723 Joules           ( -ve sign indicates heat given

dH = - 3230.5 KJ/mol               

workd done = 0          since we had constant volume process and work done at constant volume - 0

dU = Q + W =   - 31723 J = -31.723 KJ