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 5.28 g of NH4Br(s) are dissolved in 111.10 g of water, the temperature of the solution drops from 23.36 to 21.24 °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 NH4Br(s) in kJ/mol. Assume the specific heat of the solution is equal to the specific heat of water. Hdissolution =________ kJ/mol

Explanation / Answer

Solution :-

5.28 g NH4Br

Mass of water = 111.10 g

T1 = 23.36 C and T2 = 21.24 C

Delta T = 23.36 C – 21.24C = 2.12 C

C cal = 1.64 J /C

Delta H dissolution = ?

Lets first calculate the amount of the energy absorbed by the 5.28 g NH4Br

q = (m*c water * delta T) + (C cal * delta T)

q= (111.10 g * 4.184 J per g C * 2.12 C) + (1.64 J per C * 2.12 C)

q= 989 J

So 5.28 g NH4Br needs 989 J heat

Now lets calculate for the 1 mole of the NH4Br

Molar mass of NH4Br = 97.9425 g per mol

So lets calculate the Delta H solution per mole

989 J * 97.9425 g per mol / 5.28 g = 18346 J / mol

So we can convert it from joules to kJ as follows

Delta H dissolution = 18346 J per mol * 1 kJ / 1000 J = 18.35 kJ / mol

So the delta H dissolution for the NH4Br = 18.35 kJ / mol

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