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Question about avogadro's number

  1. Jun 23, 2010 #1
    I don't understand Avogadro's number when dealing with relative molecular weights.

    I understand that the relative atomic weights of the various elements are each equal to avogadro's number. But take for example water, H2O. If Avogadro's number is equal to the relative atomic weight of whatever element, then shouldn't the total number of atoms in H2O be avogadro's number multiplied by 3?

    Instead, H2O is equal to avogadro's number for some reason which I don't understand.

    Note: I'm not sure if these kinds of questions belong in this part of the forums. It's not a homework problem thought, so I dunno.
  2. jcsd
  3. Jun 23, 2010 #2


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    Science Advisor

    Avogadro's number is simply a number, like the number 7. Atoms and molecules are so small that in order to weigh them, we have decided to weigh a large number of them so that the weight is enough to register on a scale. This is where Avogadro's number comes in - it is simply a large number that we all agree to use when we measure atomic and molecular weights, and is equal to 6.02X10^23.

    So the weight of an Avogadro's number of atoms or molecules is equal to the atomic or molecular weight of the substance. For hydrogen, an Avogadro's number of atoms weighs about 1 gram. For oxygen, an Avogadro's number of atoms weighs about 16 grams. For water, an Avogadro's number of water molecules weighs about 18 grams, since it is just one oxygen and two hydrogens. Does this help?
  4. Jun 23, 2010 #3


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    Avogadro's number is not equal to atomic weight. Avogadro's number is just a number of atoms, or molecules, or ions, anything like that.

    It is - number of atoms (diiferent atoms, some of them being H and others being O) in 1 mole of water is 3 times Avogadro's number. Number of atoms in 1 mole of H2 is 2*NA, in one mole of H2SO4 is 7*NA and so on.

    You must try to be precise. Water is not equal to Avogadro's number, just like my hand is not equal to square root of 2. I have no idea what you trying to say here, and I guess that's the part of your problem. Try to explicitly and precisely write what you mean.

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