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Homework Help: Mole concept

  1. Nov 18, 2011 #1
    1. The problem statement, all variables and given/known data

    i have just learnt that one mole is actually what i had learnt as one molecule. now one atom is also known as one mole.eg. one mole of methane has 5 moles of atoms( this sentence is written by my teacher and no matter how i try to look at it from different perspectives it just seems wrong, i can't seem to understand it.:( )

    anyway, this is one of the hw qns i have problems with.

    what is the concentration of hydrogen ions when 100cm3 of 0.20 mol/dm3 of nitric acid is mixed with 300cm3 of 0.25mol/dm3 sulfuric acid?

    2. Relevant equations
    HNO3 ---> H+ + NO3-

    3. The attempt at a solution

    can someone help me solve this and explain to me how to solve it? i'm also trying to solve it right now.>.<
  2. jcsd
  3. Nov 18, 2011 #2


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    Staff: Mentor

    One dozen of methane molecules is 12 molecules, right?

    One methane molecule - CH4 - is five atoms, one carbon and four hydrogens, right?

    So one dozen of methane molecules contains - together - 5 dozen of atoms of all kinds, right?

    Now replace "dozen" with a "mole" and repeat whole exercise.

    Please show how you approached the H+ question. It is not difficult - it is almost a direct application of the concentration definition, just both volume and number of moles are not given directly, but have to be calculated.

    Actually it IS a difficult question, but for reasons you don't know yet. Don't worry, you are expected to ignore 90% of what is really happening in the solution. At the same I would NEVER ask such a question. I hate questions that are wrong by definition.
  4. Nov 18, 2011 #3
    um ok i get the dozen thing but i dont get why i should replace "dozen"with "molë"?

    uh ok firstly i found the number of moles in the nitirc acid--- which is 1/10 dm3* 0.20mol/dm3 which is equal to 0.02 mol
    since the no. of mol in the nitric acid is 0.02, conc of h ions is 0.02 as well.

    no. of moles in sulphuric acid is 0.075 so conc of h ions is 0.15. conc of h ions altogether is 0.15 + 0.02 which is equal to 0.17??

    ok this seems wrong somehow.
  5. Nov 18, 2011 #4


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    Staff: Mentor

    Mole is an overgrown dozen. Dozen is 12 objects, mole is 6.02x1023 objects.

    You started right - you correctly calculated number of moles of H+ from both sources. However, while you can add numbers of moles, you can't add concentrations.

    You know how many moles of H+ from each source you have. Can you calculate total number of moles? Do you know final volume? Can you calculate concentration using both number of moles and volume?
  6. Nov 18, 2011 #5
    yeah i know avogadro's constant. but i still don't understand why?? ok another qns, oxygen is diatomic, but it is counted as one mole not 2 right? (ps. am i making any sense??)

    oh so i should take total no. of moles over total volume to get conc of hydrogen ions which is 0.095mol/ 0.4dm3 which gives me 0.238mol/dm3 (3s.f.)

    is this right?
  7. Nov 18, 2011 #6


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    Staff: Mentor

    No idea what the question is. Why what? Why you can replace "dozen" with a "mole"? Because both refer to the same - number of objects. Just like gross (dozen of dozens, or 144 objects) or threescore (60 objects). Numbers are different, but the idea is identical.

    It counts as one mole of diatomic molecules, or two moles of atoms. Usually when we say "mole of oxygen" we mean "mole of diatomic molecules".

    Close, but you forgot that sulfuric acid is diprotic.
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