# Gas stoichiometry questions

So iv solved all these problems, id just like someone to take a look at them and see which approach is right.

The question is in the attachment.

first approach is i use molar volume for SATP which is 24.76L and multiply 65L with it
and i get 1609.4L then i use this in the universal gas forumla(i think thats what its called)

PV=nRT

(100)(1609.4) = n(8.31)(298)

(100)(1609.4)/(8.31)(298) = n

second approach i use is where i just use the 65L of volume as is and put it in the equation, since they both are volumes, which one is the right way?

PV=nRT

(100)(65)=n(8.31)(298)
(100)(65)/(8.31)(298) = n

which approach is correct?

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Also another question here, in a 2nd attachment

v=7.5 L

t=25C or 298K

P=100Kpa

PV=nRT

(100)(7.5)=n(8.31)(298)

750/(8.31)(298) =n

750/2476.38 =nHe

0.302=nHe (helium)

mass = n X molar mass

=(0.302)(4)

=1.208

Is this in grams or Kg? the mass of helium?

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Borek
Mentor
first approach is i use molar volume for SATP which is 24.76L and multiply 65L with it and i get 1609.4L
No, you don't get 1609L, you get 1609L2 - whatever it means. Watch your units and you will see when your approach doesn't make sense.

Borek
Mentor
Is this in grams or Kg? the mass of helium?

What are units of molar mass?

What are units of molar mass?
g/mol?

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No, you don't get 1609L, you get 1609L2 - whatever it means. Watch your units and you will see when your approach doesn't make sense.
so the first approach is what will get the correct solution?

Borek
Mentor
g/mol?
You havew to guess, you can't check?

so the first approach is what will get the correct solution?
Only if the units make sense. Check if they do. What units should the final answer be in? What units your final answer has?

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i did check but i wasnt sure, hence the question mark.