Homework Help: Chemistry I

Posted by Lindsey on Tuesday, November 1, 2011 at 1:11am.

Instant cold packs, often used to ice athletic injuries on the field, contain ammonium nitrate and water separated by a thin plastic divider. When the divider is broken, the ammonium nitrate dissolves according to the following endothermic reaction:

In order to measure the enthalpy change for this reaction, 1.25 of is dissolved in enough water to make 25.0 of solution. The initial temperature is 25.8 and the final temperature (after the solid dissolves) is 21.9 .
Calculate the change in enthalpy for the reaction. (Use as the density of the solution and as the specific heat capacity.)

Chemistry I - DrBob222, Tuesday, November 1, 2011 at 2:32pm
1.25 what?
25.0 what?
25.8 what?
21.9 what?
Also the sentence in parentheses omits the density and specific heat of water to be used.
The following is how the problem is solved after you plug in the right units.
q = mass H2O x specific heat H2O x delta T
q/grams = delta H/gram and
(delta H in J/g) x molar mass NH4NO3 = delta H in J/mol.

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