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Homework Help: A few basic questions

  1. Apr 8, 2007 #1
    Hi everyone

    I've just got a few questions regarding some Chemistry stuff I've been doing out of my textbook (Year 11 Chemistry). We've been learning this in class, I thought I understood it, but I must be blacking out a little bit.

    5) Calculate the mass of one atom of:

    a) Zinc


    65.4 AMU.

    In one mole of Zn there is 65.4g/ mol.

    How do I go from here? I thought it would've been something like 1 x 6.02x 10 (23), but that would show the amount of atoms in total, and if I'm trying to find the mass of one atom, how do I go about it? do I divide by the atomic weight? am I doing something wrong?

    Also, the molar mass and molecular weight, is that just the same as the mass of one mole of [whatever]?


    10) What is the mass of:

    c) 6.24 x 10-2 mol hydrogen gas?

    H x 1 + 0 x 2

    1 + 32

    = 33 AMU.

    So in one mole of H02 there is 33g/ mol.

    6.24 x 10-2

    Am I on the right path?

    Is Molecules the same thing as Atoms? I often get confused with moles and molecules, but understand they are 2 different things.

    11) How many molecules are there in 7.0g of nitrogen gas, N2? How many nitrogen atoms are there in this sample?

    N x 2
    14 x 2
    = 28 AMU

    Therefore one mole of N2 has a mass of 28g/ mol.


    = 0.25 mol.

    0.25 x 6.02 x 10 (23)

    Please point out if I'm doing anything incorrectly.

    I don't expect answers for the questions, as they aren't really homework questions, but I'd really like to know how to do them, as it's holidays and I haven't got a teacher handy. I've given all the questions a go, to the best of my ability and knowledge.

    Thanks so much.
    Last edited: Apr 9, 2007
  2. jcsd
  3. Apr 9, 2007 #2
    anyone? :confused:
  4. Apr 9, 2007 #3
    You are on the correct path with 5. Remember to keep your units in the equations and cancel as you go, it helps you to see if you are on the right track.
    Atomic mass and molar mass are generally used interchangeably, they have the units grams/mol, as in 65.4 g of Zn will contain about 6.02x10^23 atoms.
    Molecular mass is when the molar mass of all of the atoms in the molecule are added up.


    mm | moles

    moles | avagad #

    so g/mm = moles
    moles x avagad # = atoms
    Last edited: Apr 9, 2007
  5. Apr 9, 2007 #4
    For the first one, since 1 mol contains 6.023x10^23 atoms and weighs 65 gms, can you find the weight of one atom?

    In the second one, they've asked for H2 gas, not H20. Again, 1mol=2gms, 6.24 x 10-2 mol hydrogen gas=?

    Your last calculation is correct. In this question, the solution you get corresponds to the number of N2 MOLECULES. Now, each MOLECULE contains 2 nitrogen ATOMS. Therefore, the number of atoms is TWICE the number of molecules.

    Molecule refers to the whole unit. Atom refers to the individual components of that unit. Like 1 molecule of H2SO4 contains 2 atoms of H 1 of S, etc. The same way, 1 mole of H2SO4 will contain 2 moles of atoms of H and 2 moles of atoms of S and 4 moles of atoms of O. Does that help?
  6. Apr 9, 2007 #5
    Thanks, I've understand all but one, I think I might have it right, but I just want to confirm.

    Seeing as it's the mass (weight) of one atom (Zn contains one atom only), I thought it would be 65.4g ? but something doesn't look right.

    Here's an alternative I came up with, please correct me.

    5) Calculate the mass of one atom of:

    a) Zinc

    Zn x 1

    65.4 x 1 = 65.4 AMU.

    In one mole of Zn there is 65.4g/ mol.

    In one mole there is 6.02x 10 ^ 23 atoms.

    So, then:

    6.02 x 10 ^ 23

    = 1.09 x 10 (-22)


    To go from Moles to Molecules, do I use Avo. # ? if so, I've been doing it right, except it's confusing because, to calculate atoms, the same number is used, and isn't there twice the amount of atoms in one molecule?

    One more thing : )

    b) Sulfur Dioxide, SO2, reacts with oxygen for form sulfur trioxide SO3. How many moles of oxygen are needed to form 0.6 mole of Sulfur Trioxide.

    SO2 + O2 > SO3

    1.5(SO2) + O2 > (SO3)

    3(SO2) + 2(O2) > 2(SO3)

    I know the answer is 0.3, but I have no idea what to do?

    Last edited: Apr 9, 2007
  7. Apr 10, 2007 #6
    2SO2 + O2 -> 2 SO3, that might help.

    Youre right in what youre doing in the first part. And yeah, there are twice the number of atoms in the molecule, and the avo # helps you find the no of molecules.
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