Question

Instant cold packs, often used to ice athletic injuries on the field, contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the following endothermic reaction:
NH4NO3(s)→NH+4(aq)+NO−3(aq)
The enthalpy change for this reaction is called the heat of solution for ammonium nitrate. In order to measure this, 8.32 g of NH4NO3  is dissolved in enough water to make 40.83 mL of solution. The initial temperature is 27.80 ∘C and the final temperature (after the solid dissolves) is 21.80 ∘C.

Part A

kJ/mol

Answer 1

Density of Water = 1 gm/mL

Mass of water = Density of Water * Volume of Water = 1 gm/mL * 40.83 mL = 40.83 gm

Q = mass * specific heat of water * (Change in temperature)

=> 40.83 * 4.184 J/gC * (27.80-21.80)

=> 1024.99 J

Molar mass of NH4NO3 = 14 + 4 * 1 + 14 + 3 * 16 = 28 + 4 + 48 = 80 gm/mol

Number of moles of NH4NO3 = 8.32/80 = 0.104 moles

Heat of solution of ammonium nitrate = Heat absorbed/number of moles

=> 1024.99/0.104

=> 9855.67 J = 9.855 KJ/mol