[nikita]

Maybe this one will be easier for you guys.  Once again, Ive calculated the same wrong answer over and over again.

A calorimeter contains 18.0 mL of water at 14.5 C. When 1.40 g of X (a substance with a molar mass of 53.0g/mol ) is added, it dissolves via the reaction
X(s) + H₂O(l)  X (aq)

and the temperature of the solution increases to 27.0 C.
Calculate the enthalpy change for this reaction per mole of X.
Assume that the specific heat and density of the resulting solution are equal to those of water [4.18J/gC and 1.00g/mL ] and that no heat is lost to the calorimeter itself, nor to the surroundings.

here is what i did.
18mL * 1.0 g * 4.18 J/gC * (27.0-14.5C) = 940.5

mols of X = 1.40/53.0 = .026

-940.5/.026 = -36173J * 1kj/1000J=  -36.2kj/mol

are my sig fig off or something?  the hint i keep getting is "In the calculation of q, the total mass of reactant and water is needed. You may have used just the mass of the water."   i am not sure what that means?

[enahs]

When you add 1.4g of stuff, you are told to assume essentially it becomes water with regards to thermal properties. But the mass does not disappear into nothingness. So now you have 19.4 g of a solution, where you assume the density and heat capacity is still that of water.

[nikita]

ahhh, ok.  i dont know why i ignored that 1.40 as part of the mass, even after the message pretty much said so.   thank you.