CHM 2045C Module Seven Sample Exam ANSWERS

Module Seven Part F: Calorimeter Problems 5 points

Benzoic acid (C₆H₅COOH) is sometimes used as a standard to determine the heat capacity of a bomb calorimeter (constant volume). The calorimeter is an insulated containing with 1.20 kg of water. When 1.32 g of benzoic acid is burned in a calorimeter that is being calibrated , the temperature rises from 20.93 ^oC to 22.93 ^oC. What is the heat capacity of the calorimeter? The heat of combustion of benzoic acid (q_v) is -26.42 kJ/g

Solution: Read section 6.6 in the text. In the lab we will do a coffee cup calorimeter lab, which is a constant pressure calorimeter. This problem is a bomb calorimeter which is constant volume which is explained on pages 259-260.

ΔT = 22.93-20.93 = 2.00 ^oC

q_water = C_water x m_water x ΔT

q_water = 4.184 J/g ^oC x 1200 g x 2.00 ^oC = 10,042 J = 10.04 kJ

q_rxn = m _{benzoic acid} x ΔH_{benzoic acid}

q_rxn = 1.32 g x - 26.42 kJ/g = 34.87 kJ

q_bomb = C_bomb x ΔT

q_bomb = C_bomb x 2.00 ^oC

q_water + q_rxn + q_bomb = 0

10.04 kJ + (2.00 ^oC) C_bomb + 34.87 kJ = 0

(2.00 ^oC) C_bomb + 44.91 kJ = 0

C_bomb = - 22.46 kJ/ ^oC

On page 260 is Example 6.7 which is another worked example. Try Exercise 6.10 on page 261 as a sample Part F problem. Both constant volume and constant pressure problems are at the end of the chapter #31-#42 on pages 273-4