Instant cold packs contain solid NH_4NO_3 (24.0 g) and a pouch of water (100. mL
ID: 506800 • Letter: I
Question
Instant cold packs contain solid NH_4NO_3 (24.0 g) and a pouch of water (100. mL). When the pack is squeezed, the pouch breaks and the solid dissolves, lowering the temperature as a result of the following process: NH_4NO_3 (s) rightarrow NH_4^+ (aq) + NO_3^- (aq) delta H = +25.7 kJ/mol Calculate the final temperature (in degree C) of the cold pack after it has been squeezed. Assume a specific heat capacity of 4.18 J/(g. degree C) for the solution, an initial temperature of 25.0 degree C, no heat transfer between the cold pack and the environment, and a density of the solution = 1.0 g/mL. (a) 39.9 degree C (b) 10.1 degree C (c) 4.50 degree C (d) 0.00 degree C (e) -14.9 degree CExplanation / Answer
answer : b) 10.1 oC
moles of NH4NO3 = 24 / 80 = 0.3
delta H = 25.7
delta H = Q / n
25.7 = Q / 0.3
Q = - 7.71 kJ
heat = - 7710 J
Q = m Cp dT
- 7710 = (100 + 24) x 4.18 x (Tf - 25)
Tf = 10.1 oC
final temperature = 10.1 oC