How do I decide what mass and specific heat capacity to use to find the heat involved when I place a hot ionic solid into water? Do I use the mass of the solid or the water?
Well, the ionic solid will evidently dissolve in the water. Depending on how much ionic solid is dissolved, it may be a good approximation to simply use the specific heat capacity of water and the mass of water.
When there isn't a lot of solid, the solid particles should disperse itself amongst the water molecules (surrounded by them), minimally changing the mass (and volume) of the overall solution.
If it's, say, 5 g of solid in 100 mL of water, then it's probably fine to simply use
(When possible, you should reweigh the solution with the dissolved solid to compare the masses.)
If you use the mass of the solid, then of course, you would underestimate the mass of the solution by quite a lot. You would literally be ignoring the water, which doesn't make sense.
If, say, you have like 25 g of solid urea in 50 mL of water, then you definitely can't simply use the mass of water. Clearly, that would significantly increase the mass (and volume) of the solution.