# 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?

##### 1 Answer
Nov 27, 2016

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 ${c}_{w} = \text{4.184 J/g"^@ "C}$ and the mass of the water (assuming its density is $\text{0.997 g/mL}$ at ${25}^{\circ} \text{C}$).

(When possible, you should reweigh the solution with the dissolved solid to compare the masses.)

NOTE:

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.