Tricky question?

[myair]

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when we are dealing with volume and pv=nRT we have
V stp= v2(P2/P1)(T1/T2)

so how do we know if we have STP volume or the 2nd V2?

for example:
what is the MW of a gas if 2.5 g of it occupy 2L at 630 torr and 600 K temp?

I just don't know why we are looking for V1 and have V2 in the question? ie. V2= VSTP (P1/P2)(T2/T1)

Do I make sence?

 

[MoooShuuu]

For this question I would do PV=nRT
(630/760)(2)=n(.0821)(600)

Find n=number of moles...... n= 2.5g/(MW)


Am I wrong? Myair, I have no idea what you are talking about with the other formula.😕

 

[chessxwizard]

PV = nRT

But n = Mass / Molecular weight

PV = (m/MW)RT
MW(PV) = mRT
MW = mRT/(PV) = (m/V)(RT/P) = dRT/P

MW = dRT/P (where d is density, which you find using mass / volume)

MW = [(2.5g/2L)(.0821 L-atm/K-mol)(600K)]/[630/760 atm]
MW = 74.29 g/mol

 

[Dencology]

just know to find MW you need the following mole=mass/MW.

so from here you can MW by knowing how to find out density of gas from PV=nRT just do what chessxwizard said. nicely done.

 

[MoooShuuu]

my way works too 🙂

 

[Dencology]

my way works too 🙂

yes. yours works also.

 

[myair]

my way works too 🙂

yes mine was a very long way and I got my point clear 🙂

 

[MoooShuuu]

hey myair,

Have you done Kaplan blue book questions from the chapters? I remember seeing in one chapter with the PV/T=PV/T kinda what you were talking about in your original post. Where they took the PV/T in STP and then the given in PV/T and made the volume or something convert to V in STP before using the formula. ( I don't have the book in front of me) Is this what you were talking about??

 

[myair]

hey myair,

Have you done Kaplan blue book questions from the chapters? I remember seeing in one chapter with the PV/T=PV/T kinda what you were talking about in your original post. Where they took the PV/T in STP and then the given in PV/T and made the volume or something convert to V in STP before using the formula. ( I don't have the book in front of me) Is this what you were talking about??

yes I think I got it from there, I do not remember exactly. But your way is very much better.