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**1. Homework Statement**

10.11g of CaO (molar mass = 56.08 g / mol) is dropped into a styrofoam coffee cup containing 157.0 g of H

_{2}O at 25.0ºC. If the following reaction occurs, calculate the final temperature of the water assuming that the cup acts as a perfect insulator.

(Assume that the solution has the same specific heat capacity as water.)

CaO(s) + H

_{2}O(l) → Ca(OH)

_{2}(aq) ∆H = –64.8 kJ

m = 10.11 g of CaO + 157.0 g of H

_{2}O

∆T = (Tf - Ti) = (Tf - 25.0ºC

C

_{s}= 4.184

q = -64800 J

**2. Homework Equations**

q = mC

_{s}∆T

**3. The Attempt at a Solution**

q = mC

_{s}∆T

-64800 = (167.11g)(4.184)(Tf - 25ºC)

-64800 = 699Tf - 17480

-64800 + 17480 = 699Tf

-47320 = 699 Tf

Tf = 67.7ºC

However i know that isn't right,

The answer i was given was to be 42.8ºC

So i'm confused as to what i did wrong...