# Gas Law Help!

#### JohnDoeDDS

##### Senior Member
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Ok guys. I knoiw these questions are easy but for some reason I am really confused when I am doing them. Can someone please explain and help me out how to solve these types of questions? I would appreciate it thanks.

Ques:

What is the molecular weight of an uknown gas if 2.5g of it occupies 2L at 630 torr and a temp of 600K?

I do the P1/V1=P2/V2
Find V2.
Find the density by diving 2.5/2
Then 1.25=Mass/V2

But I never get the right answer and it takes me at least 10 minutes! So what am I doing wrong and are there any shortcuts to this? Thanks in advance.

#### DMD-2-B

##### Senior Member
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JohnDoeDDS said:
Ok guys. I knoiw these questions are easy but for some reason I am really confused when I am doing them. Can someone please explain and help me out how to solve these types of questions? I would appreciate it thanks.

Ques:

What is the molecular weight of an uknown gas if 2.5g of it occupies 2L at 630 torr and a temp of 600K?

I do the P1/V1=P2/V2
Find V2.
Find the density by diving 2.5/2
Then 1.25=Mass/V2

But I never get the right answer and it takes me at least 10 minutes! So what am I doing wrong and are there any shortcuts to this? Thanks in advance.
I think this is a PV=nRT question. Leave n as your unknown and solve for it with the given info. Then use the weight (2.5g) to get the molecular weight from the number of moles. Thats how I'd do it.

#### Zabispurs

##### Member
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did you convert torr to atm?

#### whiskeysour

##### up'n the competition
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Zabispurs said:
did you convert torr to atm?
it depends on what gas constant (R) you are using. if you want to use R=0.0821, then Yes, you have to convert it to atm.

WS

#### egots

##### Senior Member
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P1V1/T1 = P2V2/T2 P1 = 1 atm, T1= 273 k, T2= 600, P2= about 1 atm (dont need to be exact) V2=2

2.5 g / V1 (L) * (22.4 L/mol) = X g/mol

u could use PV = NRT i just would rather avoid dealing with constants cuz its a pain in the ass for me when i deal with .08

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#### Resonance

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JohnDoeDDS said:
Ok guys. I knoiw these questions are easy but for some reason I am really confused when I am doing them. Can someone please explain and help me out how to solve these types of questions? I would appreciate it thanks.

Ques:

What is the molecular weight of an uknown gas if 2.5g of it occupies 2L at 630 torr and a temp of 600K?

I do the P1/V1=P2/V2
Find V2.
Find the density by diving 2.5/2
Then 1.25=Mass/V2

But I never get the right answer and it takes me at least 10 minutes! So what am I doing wrong and are there any shortcuts to this? Thanks in advance.
I like to use the equation: density = (P x (Molar Mass)) / RT, that seems to be the fastest, just don't forget to convert torrs to atms. I hate this kaplan question, I just did it last night.

#### joonkimdds

##### Senior Member
10+ Year Member
7+ Year Member
JohnDoeDDS said:
Ok guys. I knoiw these questions are easy but for some reason I am really confused when I am doing them. Can someone please explain and help me out how to solve these types of questions? I would appreciate it thanks.

Ques:

What is the molecular weight of an uknown gas if 2.5g of it occupies 2L at 630 torr and a temp of 600K?

I do the P1/V1=P2/V2
Find V2.
Find the density by diving 2.5/2
Then 1.25=Mass/V2

But I never get the right answer and it takes me at least 10 minutes! So what am I doing wrong and are there any shortcuts to this? Thanks in advance.
I got 71.599 g/mol . you should tell me what the answer is so we can help u.

OP
J

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JohnDoeDDS said: