A. When a 0.235-g sample of benzoic acid is combusted in a bomb calorimeter, the

Question

A. When a 0.235-g sample of benzoic acid is combusted in a bomb calorimeter, the temperature rises 1.644 C . When a 0.275-g sample of caffeine, C8H10O2N4, is burned, the temperature rises 1.585 C . Using the value 26.38 kJ/g for the heat of combustion of benzoic acid, calculate the heat of combustion per mole of caffeine at constant volume.

B. Assuming that there is an uncertainty of 0.002 C in each temperature reading and that the masses of samples are measured to 0.001 g, what is the estimated uncertainty in the value calculated for the heat of combustion per mole of caffeine?

Explanation / Answer

The answer for A is

Benzoic acid

Heat capacity of the colorimeter Ccal = (q x m)/ T

q = heat of combustion of substance

m = mass in g of substance

T = Temperature change in °C

= (26.38 * 0.235)/1.644

= 6.1993/1.644

Heat capacity of the colorimeter Ccal = 3.770864 kJ/°C

Caffeine

Heat of combustion q = C x T

= (3.7708 kJ/°C)(1.585 °C)

= 5.976819 kJ

Heat of combustion per mole of caffeine = heat of combustion/moles

Finding moles of caffiene

(Moles = grams/molecular weight

Molecular weight of caffeine = 194.19 g/mol

= 0.275/194.19

= 0.001416 mol)

Heat of combustion = 5.9768/0.001416

= 4220.504 kJ/mol

The answer for B is

Temperature

Uncertainty = 0.002C

dT = +/- 0.004C

Benzoic acid = (0.004/1.644)*100 = 0.243 %

Caffeine = (0.004*1.585)*100 = 0.252%

Mass

Uncertainty = 0.001

Benzoic acid = (0.001/0.235)*100 = 0.426 %

Caffeine = (0.001/0.275)*100 = 0.364%

Total uncertainity = +/- 1.29%

1.29% of 4220.504 kJ = (4220.504/100)*1.29

= 54 kJ

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