I'm getting confused about what to use for the "M" term in q = MCAT For example: 20 g of of NaCl is poured into a coffee cup calorimeter containing 250ml of water. If the temp. inside drops by 1 degree Celcius by the time the NaCl is totally dissolved, what is the heat of solution for NaCl and water? (Specific heat of water = 4.18 J/g*DegreeCelcius Upon first thought, I was going to use 20g of NaCl as the Mass. But apparently, you're supposed to use the mass of water (250ml = 250g). Why? Ok.. now we continue and plug in etc etc.. we get -1 kJ of heat Except the answer choices are: A) -3kj/mol B)-1kj/mol C) 1KJ/mol D) 3 kJ/mol So I thought... alright.. since we used Water for the mass (for whatever reason) lets divide by the moles of Water. Wrong! We're supposed to divide by the moles of NaCl.. I am sooo confused. I thought q = mcat problems were just plug and chug.. I don't understand the thinking that goes behind them. Can someone explain pls.