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 1.85 g of KClO4(s) are dissolved in 107.30 g of water, the temperature of the solution drops from 23.37 to 21.78 °C. The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.90 J/°C. Based on the student's observation, calculate the enthalpy of dissolution of KClO4(s) in kJ/mol. Assume the specific heat of the solution is equal to the specific heat of water. Hdissolution = kJ/mol

Explanation / Answer

(4.18 J/g·°C) x (107.30 g) x (23.37 - 21.78)°C = 713.137J lost by the water

Supposing the calorimeter to be at the same temperatures as the water:
(1.90 J/°C) x (23.37 - 21.78)°C = 3.021J lost by the calorimeter

713.137J + 3.021J = 1716.158 J total lost by the combined apparatus, and gained by the solute

Hdissolution =

(0.716 kJ) / ((1.85 g of KClO4(s)) / (138.55 g KClO4(s)) /mol)) = +53.62kJ/mol-answer

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