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    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?
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    (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. :confused:
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    (Original post by Widowmaker)
    You aren't from what I can see. :confused:
    Yeah thanks that what I thought. I even took NA to 5 dp and it makes no difference. My lecturer made another mistake :eek:
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    if the gas is helium then the correct answer is 0 molecules!!!
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    (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 :eek:
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    (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
    --
    --------------

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

    Best,
    Borek
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    (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
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    Trivial, but you have got me :blushing:

    Best,
    Borek
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