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Molar Heat of KBr

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1 Answer

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!