Calculate the final temperature of the solution

[JessicaHelena]

Homework Statement

Consider the dissolution of CaCl2.
CaCl2(s)

Ca2+(aq) + 2 Cl-(aq) ΔH = -81.5 kJ
A 10.6-g sample of CaCl2 is dissolved in 109 g of water, with both substances at 25.0°C. Calculate the final temperature of the solution assuming no heat lost to the surroundings and assuming the solution has a specific heat capacity of 4.18 J/°C · g.

Homework Equations

Q=mcΔT

The Attempt at a Solution

I had thought Q=ΔH = -81.5kJ = -81.5 x 10^3 J
the mass would be 10.6+109 g
and since the solution overall has a heat capacity of 4.18 J/°Cg, I plugged the numbers in:
81.5 x 10^3 = (10.6+109)x4.18x(T_f-25)
Then I got T_f = 188 °C, which is wrong, but I don't understand why. I don't have much time until the assignment is due, so could someone please help me out? Thank you!

 

[mjc123]

How much CaCl₂ do you need to get 81.5 kJ?

 

[JessicaHelena]

@mjc123 I believe 1 mole...?

 

[mjc123]

And how many moles of CaCl₂ have you got?

 

[JessicaHelena]

@mjc123 Oh I get it now. 10.6g/M(CaCl2), multiply that by 81.5, and that's the Q value.

 

[JessicaHelena]

@mjc123 Thank you — I got a correct answer! Could please you help me with another problem I posted, though?