Biochemistry

posted by Pearl

calculate Ph occuring when 100ml of 0.10m acid is added to 500ml of 0.08m acetate buffer,pka 4.75,ph 4.5

  1. DrBob222

    I caution you that m means molality. M means molarity. I assume you meant M.
    Use the Henderson-Hasselbalch equation.
    a = acid; b = base.
    pH = pKa + log(b/a)
    4.5 = 4.75 + log b/a
    solve for b/a and I get approximately 0.6 (you should do it more accurately).
    You have two unknowns and you need a second equation. That one is
    a + b = 0.08
    Solve the two equations simultaneously. I get a = about 0.05M and b = about 0.03M.
    Now you have 500 mL of the buffer so you have how many millimoles of a and b?
    500 x 0.05 = about 25 mmoles acid.
    500 x 0.03 = about 15 mmoles base.

    ...........Ac^- + H^+ ==> HAc
    initial....15.....0.......25
    add............10(100 x 0.1 = 10 mmol)
    change....-10....-10......+10
    equil......5......0........35
    Again, these are approximate; you need to go through the calculations more accurately.
    Then plug these equilibrium concns into the HH equation and solve for pH.
    I arrived at 3.84.

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