gas molar mass question ~HELP!~

[busupshot83]

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What is the molar mass of a 2 L sample of gas that weights 8 g at a temperature of 15 degrees Celsius and a pressure of 1.5 atm?

These are the formulas you have to use:

V2 = V1 (P1/P2)(T2/T1)
d = m/V2

the answer is 63.2 g/mol, but I'm struggling to figure out why... can someone help me?

 

[Streetwolf]

Molar mass is g/mol and you have g so you need mol. You get that with the ideal gas formula:

PV = nRT

You know P, V, R, and T. So you need to find n. This would be the # of moles. Then you just take the # of grams divided by the # of moles and that's your answer. It works.

 

[busupshot83]

Thanks, I was using a method that Kaplan suggested. I guess PV = nRT is all I really need, and it is also much easier to use.

 

[Streetwolf]

But you can't use the formulas you posted because you only have one gas at one temperature, volume, pressure, and # of moles. You can only use V1/V2 or P1/P2 etc if you change one of them.

You use PV = nRT when you are at set amounts and want to come up with a missing amount.

 

[busupshot83]

But you can't use the formulas you posted because you only have one gas at one temperature, volume, pressure, and # of moles. You can only use V1/V2 or P1/P2 etc if you change one of them.

You use PV = nRT when you are at set amounts and want to come up with a missing amount.

In the Blue Book (bottom of pg. 263), it says you can use a STP gas as component "1," and use the given characteristics as component "2." Maybe I'm reading it wrong, but either way, I've figured out the problem using PV=nRT. Thanks a bunch man. 👍

 

[jackbauer!]

In the Blue Book (bottom of pg. 263), it says you can use a STP gas as component "1," and use the given characteristics as component "2." Maybe I'm reading it wrong, but either way, I've figured out the problem using PV=nRT. Thanks a bunch man. 👍

you're not at STP anyways....STP is 20 C and 1 atm

 

[busupshot83]

you're not at STP anyways....STP is 20 C and 1 atm

that's true... but STP is 25 C, isn't it? whatever, close enough.

 

[jackbauer!]

that's true... but STP is 25 C, isn't it? whatever, close enough.

STP is 273 K (20 C), 1 atm
Standard Ambiatic T & P is 298 K (25 C), 1 atm

small details. jb!🙂

 

[Streetwolf]

273 K = 0 C.

 

[busupshot83]

273 K = 0 C.

That's what I know it as too. 🙂

 

[jackbauer!]

hhaha...yea you guys were right...it was late and i was tired....i'm an AP chem teacher, i should have caught that!

jb!🙂

 

[busupshot83]

hhaha...yea you guys were right...it was late and i was tired....i'm an AP chem teacher, i should have caught that!

jb!🙂

You're an AP teacher? You must know your chem! 😎

 

[anteater85]

Molar mass is g/mol and you have g so you need mol. You get that with the ideal gas formula:

PV = nRT

You know P, V, R, and T. So you need to find n. This would be the # of moles. Then you just take the # of grams divided by the # of moles and that's your answer. It works.

thx for the answer