Instant cold packs used to treat athletic injuries contain solid NH4NO3 and a po
ID: 477042 • Letter: I
Question
Instant cold packs used to treat athletic injuries contain solid NH4NO3 and a pouch of water. When the pack is squeezed, the pouch breaks and the solid dissolves, lowering the temperature because of the following endothermic reaction.
NH4NO3(s) + H2O(l) NH4NO3(aq) H = +25.7 kJ
What is the final temperature in a squeezed cold pack that contains 50.0 g of NH4NO3 dissolved in 115 mL of water? Assume a specific heat of 4.18 J/(g·°C) for the solution, an initial temperature of 25.0°C, and no heat transfer between the cold pack and the environment. To find the mass of water use the density of water = 1.0 g/mL. Hint: The process takes place at constant pressure. 2.86 Incorrect: Your answer is incorrect. °C
Explanation / Answer
50 gm NH4NO3 = 50/80.052 = 0.6246 moles
dH = 0.6246 x25.7 = 16.052 KJ heat
let temp be T
water is cooled by 16052 J heat
16052= (4.18x115x(25-T))
16052=12017.5- 480.7T
=>-T=4034.5/480.7
=>T = -8.392 C is final temperature.