Posted by Samuel on .
If it takes three "breaths" to blow up a balloon to 1.2 L, and each breath supplies the balloon with 0.060 moles of exhaled air, how many moles of air are in a 3.0 L balloon?
i am trying to use v1/n1=v2/n2 but i am stuck, can u help? thank you

Chemistry Help 
DrBob222,
Will something like this work?
# breaths to blow 3.0 L balloon = 3 breaths x (3.0 L/1.2 L) = 7.5 breaths [or you can use the dimensional analysis method which will be 3.0
L x (3 breaths/1.2 L) =7.5 breaths.
Each breath has 0.06 moles; therefore,
0.060 moles/breath x 7.5 breaths = ?? moles 
Chemistry Help 
Samuel,
using your method i got 0.45moles,is that correct?

Chemistry Help 
DrBob222,
That was my answer, also. Check my thinking on the problem.

Chemistry Help 
Amy,
its saying that i am wrong, im stuck

Chemistry Help 
Amy,
hello?

Chemistry Help 
DrBob222,
After rereading the problem very carefully, I'm sticking with my answer. By the way, your v1/n1 = v2/n2 is exactly the same.
1.2/0.18 = 3.0/n2
n2 = 3.0 x (0.18/1.2) = 0.45
I think the problem with your answer is that the number 0.06 moles has only one significant figures in it; therefore, the answer must be rounded to one s.f. So the answer must be rounded from 0.45 moles to either 0.4 or 0.5 depending upon how your teacher has told you to round number when they end in 5. The rule I follow is to "round to the even number"; therefore, I would round to 0.4 but not everyone follows that rule. Let me know if this does or does not take care of the problem. As an interesting aside, note that if we round at the end, I would round to 0.4 BUT if I rounded at the 7.5 mark, I would round that to 8 and 8 x 0.06 = 0.48 which of course rounds to 0.5