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Question about moles

  1. Apr 23, 2012 #1
    Hi guys,
    I have been solving many stoichiometry problems but their is something I can do but I don't fully understand why it works it kinda confuses me atleast on mathematical level. I don't like doing something I don't fully understand.

    What I don't understand is if we have for example a chemical formula of burning hydrocarbons in O2.

    CxHx(g) + O2(g) ---> CO2(g) + H2O(l)

    Now what I don't understand is if we get the number of moles that is contained in water and we want after that to get the number of moles in Hydrogen that is contained inside this water why do we have to multiply by two ? didn't we already got the number of moles that is contained in water why do we have to recalculate it ? My intuition tells me that we did multiply it by 2 in order to get the number of moles that needs to react with O to form 1 mole of H2O is that correct or wrong ?


    This question may seem stupid,but I am self learning chemistry so some books doesn't explain something fully.

    Thanks.
     
  2. jcsd
  3. Apr 23, 2012 #2

    Borek

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    Staff: Mentor

    Number of moles of what? Do you mean just "number of moles of water"?
     
  4. Apr 23, 2012 #3
    I mean If we get the number of moles produced from a balanced chemical equation for H2O produced then we are asked to calculate the number of moles of hydrogen in order to get the empirical formula for hydrocarbon,then we have to calculate the number of moles hydrogen from the number of moles of water. What confuses me is why do we have to multiply it by 2 isn't already the number of moles that we calculated is of H2O has that much moles of Hydrogen in it ?
     
  5. Apr 23, 2012 #4

    Borek

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    Staff: Mentor

    If you look at the formula - H2O - there are two moles of atomic hydrogen and one mole of atomic oxygen per one mole of water. That's what these subscripted indexes mean.

    It gets slightly complicated when you think about gaseous hydrogen and oxygen, as these gases are usually diatomic, so one mole of water contains one mole of hydrogen as diatomic - but half a mole of oxygen as diatomic.
     
  6. Apr 23, 2012 #5

    phyzguy

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    A mole is just a number - like a dozen. If I have a dozen bicycles, how many bicycle wheels do I have? Two dozen right? That's the idea.
     
  7. Apr 23, 2012 #6
    @Phyzguy yeh I understand the idea of a mole, but what confuses is why getting the components of the molecule why we would have to see the multiple of each atoms after we calculated how much mole of X substance per gram of the X substance.


    @Borek
    But let's say we are given 36 g of H2O,and we want to calculate how much moles H2O contains,since its its molecular mass per mole is 18,so we have 2 moles of H2O.

    Wouldn't that 2 moles of H2O that we just calculated be the also the number of moles that is contained in H component of water this is what confuses me.

    Edit:
    About the indexes I know for example as you stated that when H2O form they form by ratio of 2 moles of Hydrogen combine and react with 1 mole of oxygen,but when we for example given the number of grams for H2O and we are asked to get how much moles we for the hydrogen didn't we already calculated the number of moles that is contained in H2O ?(thats why we calculated 2 grams of hydrogen + 16 grams of Oxygen in order to calculate the number of moles that is contained in X grams of H2O),Why we would then multiply by two to get the number of moles of the hydrogen.
     
    Last edited: Apr 23, 2012
  8. Apr 23, 2012 #7

    phyzguy

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    I don't understand what you're missing. Your 2 moles of H2O have 4 moles of H atoms and 2 moles of O atoms, just like 2 dozen bicycles have 4 dozen wheels and 2 dozen handlebars. Where is the confusion?
     
  9. Apr 23, 2012 #8
    Oh I see now I just thought 2 moles of H2O I thought that 2 moles of H2O were just one entity so I was confused of why we have in this 2 moles of of H2O 4 moles H atoms and 2 moles O. Now I get it in that 2 moles of H2O it contains also in its entities contain 4 moles of H2 and 2 moles of O.

    What confused me is that I thought this 2 moles of H2O represents both The H atoms and the O atoms I thought so why do we multiply The H atom by 2 in order to get the number of moles for the H atom. Now it makes sense thanks :).
     
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