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Basic Acid & base dissociation(pH),buffer,titration,equivalence point,moles question

  1. Feb 17, 2012 #1
    Hi guys ! I'm in serious need of help!

    Could you guys teach me how to attempt the following questions? I've tried everything..from reading up the notes(which has been trimmed along the years to make people pass)..to asking my teacher(He only explains briefly and in a way which i cant understand).
    My exams are coming soon and it'll be mostly about acid and base reaction(pH & dissociation(those low level questions)).

    I'm really at wits end and i am in seriously need of help !

    Questions

    1. Calculate the final pH of a solution if 500mL of 10M H2SO4 is mixed with 2L of 3M HCL & 30L of de-ion water.
    ( I used the titration method to find the molarity of de-ionized water( M1V1=M2V2) After that, i broke up HCL into H3O+CL- and i got lost)

    2. Calculate the number of H atoms in 0.35moles of C6H12O6.
    (i do not know how to start ..how do i apply the avogadro's rule into this type of questions?(6.02X10^23)

    3. How do u know when will be the equivalence point of a buffer?
    ( i know that strong base and strong acid buffer will give u a equivalence of 7...what about weak acid and strong base?)

    4. How do u calculate the pH if base in excess is added to an acid buffer solution?
    ( My teacher taught us only the ICE table,Buffer equation(pH=kpa+log(salt/acid)), and Ka or Kb.. is there any way which i can use along what my teacher has taught me, to tackle this type of excess buffer question?)

    Thanks ! =(
     
    Last edited: Feb 17, 2012
  2. jcsd
  3. Feb 18, 2012 #2

    Borek

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    Staff: Mentor

    Re: Basic Acid & base dissociation(pH),buffer,titration,equivalence point,moles quest

    I don't understand how you can be asked to solve other questions when you were not taught the most basic thing in whole chemistry. Sigh.

    How many molecules of C6H12O6 in 0.35 moles? How many atoms of H in one molecule? How many H atoms in total?

    (Simpler version of the same question: you have a dozen molecules of water, H2O. How many atoms of hydrogen do you have? Remember, that mole is just an overgrown dozen.)
     
  4. Feb 18, 2012 #3
    Re: Basic Acid & base dissociation(pH),buffer,titration,equivalence point,moles quest

    hahaha ikr...um i think there are like 2 H in water so there are 1.204 X 10^24 atoms?
     
  5. Feb 18, 2012 #4

    Borek

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    Staff: Mentor

    Re: Basic Acid & base dissociation(pH),buffer,titration,equivalence point,moles quest

    No, dozen molecules doesn't contain 1024 atoms. You are overcomplicating things.

    If I give you a dozen boxes, with two apples in each, how many apples do you have?

    Now replace box with a water molecule and ask about hydrogen atoms. Is it in any way different? Well, there are no boxes nor apples, but math is identical.
     
  6. Feb 18, 2012 #5
    Re: Basic Acid & base dissociation(pH),buffer,titration,equivalence point,moles quest

    Ok then,i'll get 24 apples

    A water molecule contains 2 hydrogen atoms,thus the mole of the water molecule would be 2 X the mole of the water molecule?

    but wait..isnt that the same as finding the mole of H+ ions in a water molecule?
     
  7. Feb 18, 2012 #6

    Borek

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    Staff: Mentor

    Re: Basic Acid & base dissociation(pH),buffer,titration,equivalence point,moles quest

    Good.

    You need to be precise - could be you are right, could be you are wrong, impossible to tell with your wording. I have no idea what you mean by "the mole of water molecule" - was it intended to be "mole of water molecules"?. Even if, statement "the mole of the water molecule would be 2 X the mole of the water molecule" doesn't make any sense to me. One water molecule is not two water molecules.
     
  8. Feb 18, 2012 #7
    Re: Basic Acid & base dissociation(pH),buffer,titration,equivalence point,moles quest

    Oh crap ! Sorry had a typo ! What i meant was mole of hydrogen would be 2 X the mole of the water molecules( H2O)

    i once did a question which i was asked to compare the mole of K+ between 2 different solutions of K+ions :

    Given that 2 solutions, 30mL of 0.15M K2CrO4, and 25mL of 0.08M K3PO4, find out which of the above solutions contains a greater number of moles of K+ ions.

    So what i did was to find the moles of both K2CrO4 and K3PO4. After which i multiplied the moles by 2 and 3 respectively as there were 2 and 3 K+ in each solution.

    Thus, i had since developed this theory that i can calculate the number of moles of ions simply just be multiplying the amount of ions contained, with the overall mole of a particular solution.

    Am i on the right track?
     
    Last edited: Feb 18, 2012
  9. Feb 18, 2012 #8

    Borek

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    Staff: Mentor

    Re: Basic Acid & base dissociation(pH),buffer,titration,equivalence point,moles quest

    Number of moles of atoms of hydrogen would be twice the number of moles of water molecules. OK.

    Yes, that's exactly the way to deal with the C6H12O6 problem.
     
  10. Feb 18, 2012 #9
    Re: Basic Acid & base dissociation(pH),buffer,titration,equivalence point,moles quest

    oh okay cool , thanks ! :biggrin:

    um...
    could you teach me to calculate the final pH of a solution if 500mL of 10M H2SO4 is mixed with 2L of 3M HCL & 30L of de-ion water?

    I tried the titration method to find the molarity of de-ionized water( M1V1=M2V2) After that, i broke up HCL into H3O+CL- and tried to react it with H2SO4 and got H3O+HCL+HSO4, am i right to do that?
     
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