Homework Help: Chemistry (PLS HELP ASAP)

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 mass--I 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 yuo 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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