When 0.954 g of CaO is added to 200.0 mL of 0.500 M HCl(aq), a temperature increase of 391C is observed.

Assume the solution's final volume is 200.0 mL, the density is 1.00 g/mL, and the heat capacity is 4.184 J/gC.
(Note: Pay attention to significant figures. Do not round until the final answer.)
Hrxn , for the reaction of

CaO(s) + 2H+(aq) Ca2+(aq) + H2O(l)

2 answers

We will be happy to critique your thinking. We are not inclined to do it for you.
I wonder if you have omitted a decimal point in the T increase. An increase of 391 C from almost any starting point leaves us with superheated steam and the heat capacity of steam is not the same as that of water.