4. Instant cold packs are used for first aid when ice is not available. In one s

Question

4. Instant cold packs are used for first aid when ice is not available. In one such device, a small pouch containing 2x102 grams of ammonium nitrate (NH4NO3) is placed in a bag containing 2x102 milliliters of water initially at 25oC. When the pouch is broken, ammonium nitrate dissolves in the water and the cold pack reaches a final temperature of -2oC. i). Calculate Hos (kJ/mol NH4NO3) for the solution in the cold pack, assuming that the pack loses negligible amount of heat to the surroundings during the dissolution process and that the heat capacity of the solution is that of pure water. ii). What would be the final temperature (oC) of a pack containing 300 grams of ammonium nitrate and 300 ml of water? Show work

Explanation / Answer

dHo = mCpdT/moles of NHNO3

with,

m = 204 g [density of water = 1 g/ml]

Cp = 4.184 J/g.oC

dT = -2 - 25 = -27 oC

So,

(i) dHo = 204 x 4.184 x 27 x 80.04/204

           = 9.042 kJ/mol

(ii) with 300 g NH4NO3 and 300 ml (300 g) water

let Tf be the final temperature

9042 = 300 x 4.184 x (Tf - 25) x 80.04/300

Tf = 27.31

Final temperature = -2.31 oC

Related Questions

Navigate

• Browse (All)
• Browse 4
• Subjects

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.