[jennielynn_1980]

THE QUESTION:

A calorimeter contains 16.0 mL of water at 13.5 C. When 1.80 g of X (a substance with a molar mass of 51.0 g/mol) is added, it dissolves via the reaction

 X(s)+H_2O(l)----> X(aq)
and the temperature of the solution increases to 27.0C.

Calculate the enthalpy change, Delta H, for this reaction per mole of X.

Assume that the specific heat and density of the resulting solution are equal to those of water [4.18 J/(g C) and 1.00 g/mL] and that no heat is lost to the calorimeter itself, nor to the surroundings.


MY ANSWER:
q = m x c x deltaT
   = 16.0g x 4.18 J/g C x 13.5 C
   = 902.88 J

moles of X  = 1.8g/51g mol
                = 0.0353 mol X

902.88 J/0.0353 mol = 25577.33 J/mol X
                            = 25.58 kJ/mol X

My answer is incorrect and I do not know why.  Could someone please point me in the right direction? Thanks alot

[UG]

Enthalpy change is negative since reaction is exothermic. See if that works.

[jennielynn_1980]

Thanks UG.  That was part of the problem.
I got the right answer by considering what you said and also adding the mass of X to the mass of water to get -28.4 kj/mol X