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 18.95 g of Cs2SO4(s) are dissolved in 103.50 g of water, the temperature of the solution drops from 24.13 to 21.53 °C.

The heat capacity of the calorimeter (sometimes referred to as the calorimeter constant) was determined in a separate experiment to be 1.54 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.

Hdissolution =________ kJ/mol

Explanation / Answer

mol of s2SO4 = mass/MW = 18.95/361.87 = 0.05236 mol of Cs2SO4

dT= 21.53-24.13 = -2.6

mtotal = 103.5+18.95 = 122.45 g

assume this is mostly water so Cp = 4.18J/gC

-Qrxn = Qwater + Qcalorimeter

-Qrxn = (122.45 *4.184)(-2.6) + (1.54*-2.6)

-Qrxn = -1336.064

Qrxn = 1336.064 J

HRxn = Qrxn/n = 1336.064 /0.05236 = 25516.88 J/mol = 25.51 kJ/mol

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