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Molar Heat of KBr

when a 4.00 gram sample of KBr is dissolved in water in a calorimeter that has a total heat capacity of 3.185kJ*K-1 , the temperature decreases by 0.210 K. Calculate the molar heat of solution of KBr.

Carol E. | Chemistry is my specialty!!Chemistry is my specialty!!
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Harley,

This is a question about calorimetry.  We are given a temperature change and the total heat capacity of the calorimeter.  We are also given the amount ( in grams) of KBr being dissolved.  Our goal is to calculate the enthalpy change per  mole of KBr.

First, we will calculate the enthalpy change and then convert that heat into a molar quantity.

To solve, we will use the equation

ΔH = -Ccal x ΔT  where Ccal is the heat capacity of the calorimeter and ΔT is the temperature change. Notice the negative sign in front of Ccal!

So, using the values given

ΔH = -(3.185 kJ K-1) x (-0.210 K)   Notice that ΔT is negative because we are told the temperature decreased.

ΔH = 0.669 kJ, but we need to convert to a molar quantity, so

(0.669 kJ/4.00 g KBr) x (119.0 g KBr/1 mol KBr)

Grams cancel and we get 0.669 kJ/ 0.0336 mol KBr = 19.9 kJ/mol

I hope this helps!