Posted by coug on Monday, January 11, 2010 at 10:23am.
What is the molecular weight of a gas if a 15.0g sample has a pressure of 836 mm Hg at 25.0 degrees C in a 2.00 L flask?
a)167
b)1.35
c)176
d)11.1
e)none of the above
Please explain... I need to understand how to do this. Thanks.
I know that I have to use PV=nRT, then solve for n grams/molar mass
And, I know that:
P=1.1 atm
V=2.00 L
n=?
R= I think .0821
and
T= ?
I have no idea how to change 15.0g to moles, since I do not know what the element is to look at the atomic massI know that one mole is avagadro's number, but I'm still really not sure what to do here.
Please help...

Chemistry (PLS HELP ASAP)  Dr Russ, Monday, January 11, 2010 at 10:46am
Yes this is correct approach
Use PV=nRT
if you are using R=0.082 057 46 then the units are L atm K^−1 mol^−1, so you need to use P in atm which you have done. The tempaeratue needs to be in K rather than deg C.
once you have found n (number of moles)
then the molecular mass (RMM) is found from
n=mass/RMM
n you have calculated above and mass is given in the question.

Chemistry (PLS HELP ASAP)  coug, Monday, January 11, 2010 at 11:09am
OK... so I found the temp to be 298, and the moles (n) = .0089921441, but now you are saying to find the molecular mass...what is RMM? and where do I go from here?

Chemistry (PLS HELP ASAP)  Dr Russ, Monday, January 11, 2010 at 11:23am
Sorry, MM is the molecular mass or 'molecular weight'. Molecular weight tends not to be used as it is a rather old fashioned term. Weight has units of newtons which can be confusing.
In the above if you divide 1.5 g by your figure of 0.0089921441 mole (you forgot the leading zero) you will get the molecular mass (g mol^1).
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