[Daisie]

I'm studying practice problems for an AP Chem test tomorrow, and I would really appreciate some help with this.

I'm not sure what I'm supposed to plug in for mass (m) in the formula [delta H = m x c x delta T]. Is it the mass of the solvent? Of the solution?

Ex: When a 9.55-g sample of solid NaOH dissolves in 100.0 g of water in a coffee-cup calorimeter, the temperature rises from 23.6*C to 47.4*C. Calculate Delta H (in kJ/mol NaOH) for the solution process

NaOH(s) -> Na+(aq) + OH-(aq)

Assume that the specific heat of the solution is the same as that of pure water.

So, I've got:

Delta H = 100.0 g x 4.18 J/g*C x 23.8*C = 9950 J

To convert to kJ/mol NaOH:

(9.95 kJ / 9.55 g) x (40.00 g / 1 mol) = 4.17 kJ/mol -> -41.7 kJ/mol (exothermic)

But the answer in the back of the textbook is -45.7 kJ/mol. Aaaaahhhh!!!!

Thank you so, so much.

[Borek]

100+9.55.

Everything changes temperature, not just water or just NaOH.

[Daisie]

Thanks so much. That makes sense to me, but I'd seen directly conflicting information online (i.e. just use mass of water).

Thanks again.