Gas Law Help!

JohnDoeDDS

Senior Member
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7+ Year Member
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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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 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

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did you convert torr to atm?

egots

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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

keibee82

Blue_tooth...
10+ Year Member
is the answer 74.3g?

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

JohnDoeDDS

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7+ Year Member
answer is 74.1 g/mol

dat_student

Junior Member
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JohnDoeDDS said:
answer is 74.1 g/mol
n = 2.5 / MW

pv = nRT
pv = (2.5/MW) * RT

(630/760) atm * 2 L = (2.5/MW) * 0.0821 * 600 K

MW = ~74.2