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 (qv) 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

 

qwater = Cwater x mwater x ΔT

 

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

 

 

qrxn  = m benzoic acid  x  ΔHbenzoic acid

 

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

 

 

qbomb = Cbomb x   ΔT

 

qbomb = Cbomb x   2.00 oC

 

 

qwater +   qrxn  +   qbomb   =  0

 

10.04 kJ  +  (2.00 oC) Cbomb    +  34.87 kJ  =  0

                    (2.00 oC) Cbomb    +  44.91 kJ  =  0

 

                                    Cbomb  =  -  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