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Homework Help: PH and Concentration

  1. Jul 14, 2006 #1

    I need some help with a few problems. Thanks!

    (1) You have a pH Electrode that reports a voltage in
    mV, which is proportional to the hydrogen ion concentration
    in solution. You also have pH buffers at known pH of say
    4, 7 and 10. What do you do to translate the mV reading
    into pH values for any given solution with pH between
    4 and 10?

    (2) You have DI water plus CaCl2 and NaHCO3 salts.
    What do you do to prepare a solution with Ca concentration
    of 10 mmol/l and total C concentration of 2 mmol/l?
  2. jcsd
  3. Jul 14, 2006 #2


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    Hi there kawaiitora and welcome to PF,

    What are your thoughts on the problems?
  4. Jul 14, 2006 #3
    Thanks for the quick reply. Sorry, I forgot about that.

    (1) I'm thinking that I should use the Nernst equation, but I'm not sure which variation would be the best.

    (2) For this one, I'm really not sure how to start. would I need to make up a molecular equation first?
  5. Jul 14, 2006 #4


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    No problem, lets take each problem in turn.

    Question One

    You should first start with the basic Nernst Equation;

    [tex]E = E^{\theta} - \frac{RT}{nF}\cdot\ln Q[/tex]

    Now, how many moles of electrons much be transferred to balance the charge on Q (which in this case is the hydrogen ion)? If we also assume standard conditions we have sorted the [itex]\frac{RT}{nF}[/itex] coefficient. Now, the next thing you should do is convert the natural log into a log to the base 10; this involves a little bit of mathematics but is petty straight forward. Next, you will want to express Q in terms of hydrogen ion concentration or activity. After which, all is plain sailing :smile:. Can you go from here?
  6. Jul 14, 2006 #5
    Yep. Thanks, Hootenanny! Now, I need some help with the second problem....
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