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### Quiz 1 Preparation: Question 6

Posted: Thu Jan 26, 2017 5:37 pm
What mass of propane, C3H8(g), must be burned to supply 2580. kJ of heat? The standard enthalpy of combustion of propane at 298. K is -2220. kJ/mol-1.

How do I go about answering this question?

### Re: Quiz 1 Preparation: Question 6

Posted: Thu Jan 26, 2017 10:06 pm
Hi, I'm not sure if I got the right answer but what I did was:

First, looking at the problem, we know that we are have an exothermic reaction, because the propane is burned to release 2580kJ (which means delta H of this will be negative) of energy in the form of heat(q). Then, we can see that there is a combustion reaction taking place because the propane is being burned.

Now, they provide us the Standard Enthalpy of Combustion as -2220 kJ mol^-1, which is the amount of energy burning 1 mol of propane gives off. (hence the mol ^-1)

Knowing this we can set up (to find how many mols of propane must be burned to release -2580kJ of energy?)
(-2220kJ mol^-1)(x mol) = (-2580kJ) --> (-2580kJ/-2220kJ) = 1.16mol

Now that we know that 1.16mol of propane is necessary, we can calculate its molar mass by looking at the periodic table to find that its 44.097g mol^-1.

Therefore we multiply the 44.097g mol^-1 and 1.16mol to cancel out the mols and find g, which I found to be 51.15252 which we round to 51.2 to have the correct number of sig figs.

I'm not sure if this is the right answer though, this is how I solved it though

### Re: Quiz 1 Preparation: Question 6

Posted: Thu Jan 26, 2017 11:26 pm
I'm also not completely sure, but I solved it the same way as Joseph.

### Re: Quiz 1 Preparation: Question 6

Posted: Fri Feb 03, 2017 1:05 am
Are we assuming that the 2580 kJ of heat is negative because it's an exothermic reaction?

### Re: Quiz 1 Preparation: Question 6

Posted: Fri Feb 03, 2017 2:46 pm
Yes, since it releases heat it must be exothermic so the sign is negative.