chemistry help me understand please?

Question

when a 25.7-g sample of NaI dissolves in 80.0 g of water in a calorimeter, the temperature rises from 20.5 degrees celcius to 24.4 degrees celcius. calculate Delta heat for the process

Answer 1

Heat capacity of water is well known: 1 Cal/g-K.
If we make a crucial approximation that the NaI solution is considered as pure water, this problem is very easy:
(Answer) = (80.0 + 25.7)*(24.4 - 20.5) (Cal)
However, this is not a good approximation -- with 25.7g NaI in 80.0 g of water, the concentration is quite high -- the heat capacity can not be 1 Cal/g-K any more.
I wonder if there is any one who would know the true heat capacity of the solution.