Hello, Julissa,
There are several steps we need to take to answer this question. Please share what it is you'd like to focus on in future questions. I'll summarize each step, but if you have questions about a particular point, please ask.
We need a balanced equation to start. We are decomposing BrF3 into flourine and bromine. Both are diatomic elements, so we need to write them with a subscript of 2:
BrF3 = Br2 + F2
To balance this, we need 2 BrF3, so that we can form 1 Br2 .
2 BrF3 = 3F2 +Br2 .
We want 6.58 liters of fluorine gas. That gas is at 0 C and 1 atm pressure. Any other time I'd be calculating the number of moles of a gas with the ideal gas law, PV=nRT. But it is a pesky calculation, requiring I always look up the gas constant, R, and getting all the units typed in properly. And the calculation, while straightfoward, must be done carefully to avoid errors.
Soooo . . o . o . o . . ., we'll take notice that, in this case, the conditions are at STP, Standard Temperature (0 C)and Pressure (1 atm). OK!!! That means I could, and most certainly will, take a nifty shortcut to calculating moles of F2 we need to make 6.58 liters of the nasty stuff. That's because I memorized the nice fact that one mole of ALL gases at STP is 22.4 liters, or 22.4 liters/mole. Remeber it - it comes in handy many times.
Since we need 6.85 liters (at STP), we'll want to produce
(6.85 liters)/(22.4 liters/mole) = 0.294 moles F2.
The balanced equation says we get 3 moles fluorine for every 2 moles bromine trifloride. So we'll need
(2 moles BrF3/3 moles F2)*(0.294 moles F2) = 0.196 moles BrF3
Whoa, that's what the question wanted. When I started typing, I thought Id be using the ideal gas law, but it went more quickly than I expected.
Bob
