I need help in G.Chem

[Clovers]

These types of problems are driving me crazy. Hope you guys can help me out. Here is an example from Kaplan blue book:

1. What is the molecular weight of an unknown gas if 2.5g of it occupies 2L and 630 torr and a temperature of 600K?

You have to find the volume at STP first,

2L x (630 torr/760torr) x (273K/600k) = .754 L @ STP

So my question is why do you put 630 as the numerator and not denominator, and why 273 as numerator and not denominator? Other similar problems switch them around. Thanks all!

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[Clovers]

someone help please...

 

[joonkimdds]

someone help please...

i think i know how to solve this problem, but i can't think of it now lol.
and i am studying for my organic
someone help this guy~
anyway here is my BUMP!

 

[erichen]

These types of problems are driving me crazy. Hope you guys can help me out. Here is an example from Kaplan blue book:

1. What is the molecular weight of an unknown gas if 2.5g of it occupies 2L and 630 torr and a temperature of 600K?

You have to find the volume at STP first,

2L x (630 torr/760torr) x (273K/600k) = .754 L @ STP

So my question is why do you put 630 as the numerator and not denominator, and why 273 as numerator and not denominator? Other similar problems switch them around. Thanks all!

since to find molecular weight, it will be convenient to know how many moles you are dealing with.

PV=nRT

so convert pressure from torr to atm because atm is the standard pressure unit for the ideal gas law: 670 torr= (670/760) atm, or 0.8158 atm (since 760 torr= 1atm)

(you are finding how many atms 670 torr equals to, thats why 670 is the numerator)(if you are still unclear, look at the units: 670 (torr) / 760 (torrs per atm) -> torrs cancel out, flip atm to the top, yielding the answer in atms)

(0.8158 atm)(2L)= n (0.08206 <- constant R) (600 K)
which roughly equals to 0.03314 moles of gas

if 2.5 g gives 0.03314 moles of the gas, then the molecular weight can be related as 2.5g /(molec weight)= 0.03314 moles. solving for molec weight yields= 75.4375 g/mol gas

i hope i did this correctly

hope this helps

**[i'm not sure why they devided 273/600; probably to find the proportionality, but i don't think PV=nRT has to be at STP]

 

[Clovers]

since to find molecular weight, it will be convenient to know how many moles you are dealing with.

PV=nRT

so convert pressure from torr to atm because atm is the standard pressure unit for the ideal gas law: 670 torr= (670/760) atm, or 0.8158 atm (since 760 torr= 1atm)

(you are finding how many atms 670 torr equals to, thats why 670 is the numerator)(if you are still unclear, look at the units: 670 (torr) / 760 (torrs per atm) -> torrs cancel out, flip atm to the top, yielding the answer in atms)

(0.8158 atm)(2L)= n (0.08206 <- constant R) (600 K)
which roughly equals to 0.03314 moles of gas

if 2.5 g gives 0.03314 moles of the gas, then the molecular weight can be related as 2.5g /(molec weight)= 0.03314 moles. solving for molec weight yields= 75.4375 g/mol gas

i hope i did this correctly

hope this helps

**[i'm not sure why they devided 273/600; probably to find the proportionality, but i don't think PV=nRT has to be at STP]

Hey, thanks for helping me out. The way Kaplan had in the blue book really confuses me. I get it now =)

 

[busdent]

These types of problems are driving me crazy. Hope you guys can help me out. Here is an example from Kaplan blue book:

1. What is the molecular weight of an unknown gas if 2.5g of it occupies 2L and 630 torr and a temperature of 600K?

You have to find the volume at STP first,

2L x (630 torr/760torr) x (273K/600k) = .754 L @ STP

So my question is why do you put 630 as the numerator and not denominator, and why 273 as numerator and not denominator? Other similar problems switch them around. Thanks all!

The key to doing these problems is SPEED. There a few ways to go about doing this problem but remember you have limited time and you have to do it very fast. So it pays to know some formulas

(P)(MW)= dRT

P= pressure in atm
MW=Molecular Weight
d = Density (g/L)
R = constant .0821
T = temperature in Kelvin

so now all you have to do is plug and chug to get the molecular formula.

(630/760)(MW) = (2.5g/2L)(.0821)(600K)

and just solve for MW and you get about 75 .