When a solid dissolves in water, heat may be evolved or absorbed. The heat of di
ID: 502617 • Letter: W
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 5.22 g of CsClO4(s) are dissolved in 100.70 g of water, the temperature of the solution drops from 23.33 to 20.10 °C.
The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.64 J/°C.
Based on the student's observation, calculate the enthalpy of dissolution of CsClO4(s) in kJ/mol.
Assume the specific heat of the solution is equal to the specific heat of water.
Hdissolution = _____kJ/mol
Explanation / Answer
Heat lost by water Q = mcdT c = heat capacity of water
= (100.7 g) x (4.18 J/g·°C) x (23.33 - 20.10)°C
= 1359.6 J
Supposing the calorimeter to be at the same temperatures as the water:
Heat lost by calorimeter = heat capacity of calorimeter x temp difference
= ( 1.64 J/°C) x ((23.33 - 20.10)°C
= 5.3 J
Hence,
Heat gained by solute = total heat lost by combined apparatus
= 1359.6 J + 5.3 J
= 1364.9 J
Then,
enthalpy of dissolution of CsClO4(s) = 1364.9 J / moles of CsClO4(s)
= 1364.9 J/ 5.22 g / 232.36 g/mol [ moles = mass/ molar mass ]
= 60756 J
= 60.756 kJ/mol
Therefore,
Hdissolution = 60.756 kJ/mol
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