1. When 1.550 g of liquid hexane (C6H14) undergoes combustion in a bomb calorime

Question

1. When 1.550 g of liquid hexane (C6H14) undergoes combustion in a bomb calorimeter, the temperature rises from 25.87C to 38.13C. Find Erxn for the reaction in kJ/mol hexane. The heat capacity of the bomb calorimeter, determined in a separate experiment, is 5.73 kJ/C. Express your answer in kilojoules per mole to three significant figures.

2. The combustion of toluene has a Erxn of –3.91×103 kJ/mol. When 1.55 g of toluene (C7H8) undergoes combustion in a bomb calorimeter, the temperature rises from 23.12C to 37.57C. Find the heat capacity of the bomb calorimeter. Express the heat capacity in kilojoules per degree Celsius to three significant figures.

Explanation / Answer

1.
Erxn = -C * delta T
          = - 5.73*(38.13 - 25.87)
          = -70.25 KJ

number of moles = mass/molar mass
           = 1.55/ (6*12 + 14*1)
           =0.018 mol

Erxn = -70.25 KJ / 0.018 mol
          = -3.90*10^3 KJ/mol

2.
number of moles = mass/molar mass
           = 1.55/ (7*12 + 8*1)
           =0.017 mol

Erxn = Erxn per mol * number of moles
         = -3.91*10^3*0.017
         = -65.875 KJ

Erxn = C * delta T
-65.875= -C*(37.57 - 23.12)
C = 4.56 KJ/oC

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