- Joined
- Dec 26, 2006
- Messages
- 951
- Reaction score
- 0
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.
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.