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 18.53 g of Cs2SO4(s) are dissolved in 100.40 g of water, the temperature of the solution drops from 25.54 to 22.92 °C.
The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.85 J/°C.
Based on the student's observation, calculate the enthalpy of dissolution of Cs2SO4(s) in kJ/mol.
Assume the specific heat of the solution is equal to the specific heat of water.

Explanation / Answer

The 18.53-g of Cs2SO4 caused the temperature of the 100.40-g of water to decrease by (25.54 oC - 22.92 oC)2.62 oC. In addition to the water, the calorimeter also gained heat at the rate of 1.85-J/ oC.

Heat lost = [100.40-g x 2.62 oC x 4.184-J/g*oC] + [1.85J/oC x 2.62 oC]
= 1100.59-J + 4.847-J = 1105.4398 -J

This quantity of heat was gained by 18.53-g of Cs2SO4. ---> 18.53-g MgCl2 x 1 mol /361.87 g Cs2SO4 = 0.0512 mol

Enthalpy of dissolution : 1105.4398-J/0.0512 mol = 21587.993 -J = 21.89 -kJ/mol

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