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chem

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how many mL of .100 M of acetic acid and .100 M of NaOH, would be need to make the volume 1.0 liters and have a pH7.00 if Ka=1.8x10^-5

please write out the steps or eq for me im gettin stuck

  • chem - ,

    This is not a good question because the pH of 7.00 is too far from the pKa value to work well as a buffer. However, here is the problem.
    pH = pKa + log (base)/(acid)
    7.00 = 4.74 + log (B/A)
    Solve for (base/(acid). I obtained 181.97 but you need to confirm that. Here base is referring to acetate ion (not NaOH) and acid is referring to acetic acid. I call those HAc for acetic acid and Ac^- for acetate (the base).

    Then you set up another equation.
    moles base = 181.97*moles acid
    For moles base substitute mLbase x 0.1 M. On the right side, substitute for moles acid (1000 - mLbase)*0.1 M
    Solve for mLbase
    I get something like 994.53 for base and 1000-994.53 = 5.47 mL acid. You need to confirm both numbers AND you may need to carry out to more places that you are allowed under significant figure rules BECAUSE it is such a large number for one and a small number for the other one.
    What this means is that you need to add that many mL of 0.1 M NaOH to produce that much Ac^- and since it is neutralizing HAc, the HAc is being used up at the same time. To check if this is correct,
    (Ac^-) formed = 994.53 mL x 0.1M/1000 mL = 0.099453 M.
    (HAc) remaining = 5.465 mL x 0.1 M/1000 = 0.0005465
    pH = 4.74 + log (0.099453/0.0005465) =
    pH = 4.74 + 2.26 = 7.00. I reiterate this is a poor problem AND that I have used more significant figures than allowed. With such a disparity in numbers, that is necessary to make the numbers come out right.

  • chem - ,

    I don't think this would be correct because The solution you are proposing would have a pH over 12.

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