When a solid dissolves in water, heat may be evolved or absorbed. The heat of di

Question

When a solid dissolves in water, heat may be evolved or absorbed. The heat of dissolution (dissolving) can be determined using a coffee cup calorimeter. In the laboratory a general chemistry student finds that when 4.08 g of NH_4CIO_4(s) are dissolved in 111.40 g of water, the temperature of the solution drops from 24 39 to 21.73 degreeC. The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.59 J/degreeC. Based on the student's observation, calculate the enthalpy of dissolution of NH_4CIO_4(s) in KJ/mol. Assume the specific heat of the solution is equal to the specific heat of water.

Explanation / Answer

q = m * s * dT + Cp*dT

= 111.4 * 4.184 * (24.39 - 21.73) + 1.59*(24.39 - 21.73)

= 1244.05 J

4.08 grams of NH4ClO4 absorbs energy ----> 1244.05 J

117.5 grams of NH4ClO4 absorbs energy ---> (1244.04*117.5/4.08) = 35827 J/mol = 35.827 KJ/mol

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