Before we turned in our homework, our professor pointed out that one of the answers in the back of the book was incorrect (we turn in our homework for completion and use the answers to make sure we are doing the problems correctly). Now I am studying for the test, and I keep getting a different answer than the one he gave us. In fact, I keep getting the answer in the back of the book. So I am wondering, is the book or my professor wrong? Normally I would think that the book is, but I would like to be sure.

Here is the question...

When a 9.55g sample of solid sodium hydroxide dissolves in 100.0g 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.

Our professor said that the answer should be -41.7 kJ/mol, and the answer in the back (and the answer that I keep getting) is -45.7kJ/mol. I realized that you get -41.7kJ/mol when you use 100.0g as the mass of the solution, and you get -45.7 when you use 109.55g as the mass of the solution. It didn't make sense to me to use just the mass of water and leave out the mass of sodium hydroxide when both make up the total mass of the solution. Am I missing something?