Question

# A coffee cup calorimeter initially contains 125g of water at 24.2oC. 10.5g of potassium bromide also...

A coffee cup calorimeter initially contains 125g of water at 24.2oC. 10.5g of potassium bromide also at 24.2oC is added to the water. After the KBr dissolves the final temperature is 21.1oC. Calculate the enthalpy change for dissolving the salt in J/g and kJ/mol. Assume specific heat of solution is 4.18J/goC.

Sol :-

Total mass of KBr solution (m) = 125 g + 10.5 g = 135.5 g

Heat evolve (q) = Mass of solution (m) x Heat capacity of water (C) x Change in temperature (ΔT)

= (135.5 g).(4.18 J/g.0C).(21.1-24.2)0C

= - 1755.809 J

Now, enthalpy change (ΔH) per gram of KBr solution = -1755.809 J/135.5 g

= -12.958 J/g

As, Number of moles of KBr = Given mass of KBr/Gram molar mass of KBr

= 10.5 g / 119.002 g/mol

= 0.0882 mol

So,

Enthalpy change (ΔH) per mole of KBr solution = - 1755.809 J / 0.0882 mol

= - 19907.13 J/mol

= - 19.907 KJ/mol

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