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

Question

Instant cold packs are used for first aid when ice is not available. In one such device, a small pouch containing 2.00 Times 10^2 grams of ammonium nitrate (NH_4NO_3) is palced in a bag containing 2.00 Times 10^2 millimeters of water initially at 25 degree C. When the pouch is broken, the ammonium nitrate dissolves in the water and the cold pack reaches a final temperature of -2 degree C. Calculate Delta H_s for the solution in the cold pack, assuming that the pack loses a negligible amount of heat to the surroundings during the dissolution process and that the heat capacity of the soloution is that of pure water. What would be the final temperature of a pack containing 300 grams of ammonium nitrate and 300 Ml of water?

Explanation / Answer

A) delta Hs = mass*specific heat capacity*change in temperature.

Let the density of the water be 1 g/ml

Thus, mass of the solution = mass of water + mass of NH4NO3 = 200 + 200 = 400 g

specific heat of the solution = 4.184 J/g/0C

change in temperature = -27 0C

Thus, delta Hs = 400*4.184*-27 = -45187.2 J = -45.1872 kJ

B) delta Hs = mass*specific heat*change in temperature

or, 451872 = 600*4.184*change in temperature

or, change in temperature = 18

Thus, final temperature = 25 - 18 = 7 0C

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