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Thread: moles

  1. #1 moles 
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    Hi, what i want to know is this;
    how does size of particles affect the molar mass of a substance, which in turn affects the amount of the substance that will contain 6 * 10^23 particles??
    thanks a lot!


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  3. #2 Re: moles 
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    Quote Originally Posted by DesertRose
    Hi, what i want to know is this;
    how does size of particles affect the molar mass of a substance, which in turn affects the amount of the substance that will contain 6 * 10^23 particles??
    thanks a lot!
    Size (I assume you mean volume) has no effect. Avogadro's number is a ratio of masses.


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  4. #3  
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    I think I understand what the poster is confused about.

    This is what I think they are confused about.

    They see a Hydrogen atom, it weighs 1 AMU and has 1 proton and 1 electron.
    They know a Mole of hydrogen weighs 1 gram and has 6.022 x10^23 atoms.

    They then look at something, lets say Lead and see that it weighs 207 AMU, has 82 protons and 82 electrons.
    They reason that the mass from the additional 81 protons and 126 neutrons makes this a larger atom. As such it would take up more space and thus Avogadro's number would not be as large.

    It is true that as the elements get heavier so too does the weight of one mole of that element. A mole of Lead weighs 207 grams, compared to a mole of hydrogen where a mole of it weighs 1 gram.

    However one mole, is always equal to 6.022 x10^23. That doesn't change, even when you use compounds a mole of that compound is still 6.022 x10^23 of that compound.
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    Having re-read the OP a few times I might have made a wrong assumption about the question. I will leave the information above just in case I was wrong about being wrong about the question.

    The size of an atom increases by adding protons, neutrons, and electrons to it. Doing this makes new elements. We typically ignore electron weight in the calculations because they weigh so little, so it really comes down to protons and neutrons.

    Protons and Neutrons both weigh 1 amu. When you convert from amu into grams, to get the mass of a mole of substance, you simply add the amu of the atoms involved in the compound (assuming its a compound or ignore this step for elements) then change the sign from amu to g.

    Water, H2O has an amu of approx 18. One mole of water has a mass of approx 18 grams.

    So if you have two elements lets say Carbon and Lead, and you have one mole of both of these elemetns, they would vary in a few ways. First off, the Lead would be more massive than the Carbon, by quite a lot. I don't know if it would take up more volume than the carbon, perhaps someone who knows more could answer that, but if I had to venture a guess I would say that it would occupy more volume.

    So that's basically how it works. As the protons and neutrons increase, the size and mass of the atom increases. As those increase, the mass of a mole of that substance increases.

    I hope one of my two answers is what you were seeking, or at the very least helped someone else possibly grasp your question better than I and can provide a better response.
    Always minimize the variables.

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  5. #4 Re: moles 
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    Quote Originally Posted by DesertRose
    Hi, what i want to know is this;
    how does size of particles affect the molar mass of a substance, which in turn affects the amount of the substance that will contain 6 * 10^23 particles??
    thanks a lot!
    A mole of a compound is 6 * 10^23 molecules of that compound. The mass of a mole is 6 * 10^23 times the mass of the molecule. So the molecular weight determines the molar mass.
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  6. #5  
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    A quick note on the size of atoms:
    You need to know the charge on an atom to know the size of the electron cloud it occupies. A Na atom might be a certain size, but as soon as you charge it, making it Na+, each electron will now feel a greater tug as 10 electrons have 11 protons pulling at them. As such the electron cloud size shrinks greatly. The reverse is true for fluoride gaining a negative charge.

    So the mole is dependent purely on the mass of the atom, as the volumes can vary considerably with the molecule in which it is placed, whether it is charged, etc.

    Some places it might benefit you to look for the answers to your basic chemistry questions is youtube and wikipedia.
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  7. #6  
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    Thank you very much all!
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