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# Ideal Gas Equation watch

1. 80.7 mg of a gas in a 10 L container at 25°C exerted a pressure of 5.0 kPa. How many molecules of the gas were in the container. What was the gas?

The model answer is:

pV = nRT (5 x 103 Pa)(10 x 10-3 m3) = n (8.314 J K-1 mol-1) (298 K)

n = 0.02017 moles = 1.215 x 1023 molecules

0.0807 g means the molar mass was 4 - probably He!

However when I do 0.02017 * 6.023 x 1023 = 1.215 x 1022 molecules

Where am I going wrong?
2. (Original post by supreme)
80.7 mg of a gas in a 10 L container at 25°C exerted a pressure of 5.0 kPa. How many molecules of the gas were in the container. What was the gas?

The model answer is:

pV = nRT (5 x 103 Pa)(10 x 10-3 m3) = n (8.314 J K-1 mol-1) (298 K)

n = 0.02017 moles = 1.215 x 1023 molecules

0.0807 g means the molar mass was 4 - probably He!

However when I do 0.02017 * 6.023 x 1023 = 1.215 x 1022 molecules

Where am I going wrong?
You aren't from what I can see.
3. (Original post by Widowmaker)
You aren't from what I can see.
Yeah thanks that what I thought. I even took NA to 5 dp and it makes no difference. My lecturer made another mistake
4. if the gas is helium then the correct answer is 0 molecules!!!
5. (Original post by charco)
if the gas is helium then the correct answer is 0 molecules!!!
Thanks very much for pointing that out, once I found the first error I went onto another question. At least this lecture does provide answers........even if they are sometimes wrong.....the others refuse
6. (Original post by supreme)
n = 0.02017 moles = 1.215 x 1023 molecules
However when I do 0.02017 * 6.023 x 1023 = 1.215 x 1022 molecules
Either either - both can't be OK. Where did you get first number from?

Best,
Borek
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(Original post by charco)
if the gas is helium then the correct answer is 0 molecules!!!
Could you elaborate?

Best,
Borek
7. (Original post by Borek)
Could you elaborate?
Definition of a molecule (Webster)

Molecule \Mol"e*cule\, n. [Dim. fr. L. moles a mass: cf. F.
mol['e]cule. See 3d Mole.]

(Chem.) A group of atoms so united and combined by chemical affinity that they form a complete, integrated
whole, being the smallest portion of any particular
compound that can exist in a free state; as, a molecule of
water consists of two atoms of hydrogen and one of oxygen.
Cf. Atom.

----------------------------------------------

Helium, as single atoms, hence contains no molecules!

best
Charco
8. Trivial, but you have got me

Best,
Borek
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