Question

A 5.80 g sample of solid NH4Br (s) is dissolved in 117 mL of water in a coffee cup calorimeter. Once all of the NH4Br (s) is dissolved in the water, the final temperature of the solution is found to be 6.73°C. If the initial temperature of the water in the calorimeter was 21.63 °C, calculate the calorimeter constant (in J/K) for the coffee cup calorimeter. Report your answer to three significant figures. The heat of solvation of NH4Br (s) is 16.78 kJ/mol.

Answer 1

Assuming system to be adiabatic, the heat evolved due to dissolution of NH4Br will be absrobed by the caloriemter and solution

The heat of solvation of NH4Br = 16.78 KJ / moles ( so this endothermic reaction)

The solution will require this much of energy dissolve

The moles of NH4Br dissolved = Mass / Molecular weight = 5.8 g / 98 = 0.0591

so heat absorbed (used) = 16.78 KJ X 0.0591 = 0.991KJ = 991 J

Heat lost =

= mass of water X specific heat X change in temperature + calorimeter constant X change in temperature

= 7286.9 + calorimeter constant X 14.9

Heat gained by solution = 991 Joules

calorimeter constant = 422.54 J / K