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Ammonium nitrate (MW = 80) is used in ‘cold packs’ to give a cooling effect for...

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lak-86 | Student, Undergraduate | (Level 1) Salutatorian

Posted June 12, 2013 at 7:53 PM via web

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Ammonium nitrate (MW = 80) is used in ‘cold packs’ to give a cooling effect for sports injuries. The solid crystals are added to water producing an endothermic reaction. A  typical  ‘cold pack’ contains 40  g of ammonium nitrate  that  is dissolved  in water  to make 200  g of the solution. Calculate the molar concentration of the ammonium nitrate solution and hence the drop in temperature that occurs when this pack is used.
[1 mole of ammonium nitrate dissolved  in water  to make 1 kg of  solution produces a drop in temperature of 6.2  °C]

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llltkl | College Teacher | (Level 3) Valedictorian

Posted June 13, 2013 at 12:20 AM (Answer #1)

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Ammonium nitrate dissolves in water in an endothermic reaction.

A typical ‘cold pack’ contains 40 g of ammonium nitrate that is dissolved in water to make 200 g of the solution.

We know 80 g NH4NO3 when dissolved in water to make 1000 ml solution is 1.0 M.

Therefore, 40 g NH4NO3 dissolved in water to make 200 ml solution would be 1.0*1000*40/(80*200) = 2.5 M. This is the molarity of salt in the cold pack.

Further, 1 mole of ammonium nitrate when dissolved in water absorbs sufficient heat that produces a temperature drop of 6.2 degrees Celsius of the 1 kg solution that is produced.

The typical ‘cold pack’ contains half the amount of salt in one-fifth volume of solution. Therefore, this amount of NH4NO3 upon dissolution would absorb sufficient heat to produce a temperature drop of 6.2*5/2 = 15.5 degrees Celsius.

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