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Amount of base needed to titrate to pH 10.00

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  1. Nov 22, 2014 #1
    1. The problem statement, all variables and given/known data
    A 50.0 mL sample of a 1.00 M solution of a diprotic acid
    H2A (Ka1 = 1.0 × 10–6
    and Ka2 = 1.0 × 10–10) is titrated
    with 2.00 M NaOH. What is the minimum volume of
    2.00 M NaOH needed to reach a pH of 10.00?
    (A) 12.5 mL
    (B) 37.5 mL
    (C) 25.0 mL
    (D) 50.0 mL
    2. Relevant equations
    pH=-log[H+]

    3. The attempt at a solution
    I believe I am missing something extremely obvious. I find that NaOH, being a strong base will completely neutralize the acid and then additional base will be required to have the pH reach 10.00. So I found moles of hydrogen, being .05*1.00.*2. I then divided by molarity of the NaOH to find volume of NaOH required to completely neutralize it, and find 50.0 mL. Now this is where I get confused. D is the highest answer, and only has enough base to neutralize the acid, not to dissociate and to increase pH.
     
  2. jcsd
  3. Nov 22, 2014 #2

    Borek

    User Avatar

    Staff: Mentor

    Not all acid is neutralized at pH 10.0.

    Do you know when pH=pKa?
     
  4. Nov 23, 2014 #3
    Oh wow, sorry, I'm stupid. Thanks.
     
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