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 1.01 g of Cacl2(s) are dissolved in 105.50 g of water, the temperature of the solution increases from 24.11 Thermomete to 25.83 °C. The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.81 J°C. Based on the student's observation, calculate the enthalpy of dissolution of Cacio in kJ/mol. Assume the specific heat of the solution is equal to the specific heat of water AHHdissolution kJ/mol

Explanation / Answer

Answer:

Given mass of water=105.5 g, temperature increases from 24.11°C to 25.83°C, specific heat of water=4.18J/g.°C

Heat q=m x c x (Tf-Ti)

Heat lost by water= (105.5g) x (4.18 J/g·°C) x (25.83°C - 24.11°C)= 758.503 J lost by the water

Supposing the calorimeter to be at the same temperatures as the water:

(1.81J/°C) x (25.83°C - 24.11°C)=3.113 J lost by the calorimeter

Total heat=758.503 J + 3.113 J = 761.616 J total lost by the combined apparatus.

Given mass of CaCl2=1.01 g and molar mass CaCl2=110.98 g/mol

moles of CaCl2=mass/molar mass=1.01g/110.98g/mol=0.0091mol.

Heat of dissolution for 0.0091 mol of CaCl2=761.616 J,

for 1mol CaCl2=761.616J/0.0091 mol=83687.28 J/mol=83.687 kJ/mol. (1kJ=1000J)

DeltaHdissolution=83.687 kJ/mol.

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