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March 29, 2017

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What is the new pH when 1mL of 1M HCl os added to 5mL of 0.1M Phosphate buffer (pH 7.4)?

  • Biochemistry - ,

    I assume you are using pK2 for pKa. You need to look up the value in your text or notes. My OLD text lists pK2 as 7.21.
    First,
    7.40 = 7.21 + log (base/acid)
    Solve for B/A = 1.549 which makes
    B = 1.549*A
    Then B + A = 0.1, solve these two equations simultaneously and I arrive at
    A = 0.03923M and B = 0.06077M

    5 mL of the buffer gives us
    0.03923 x 5 mL = 0.196 mmoles A
    0.06077 x 5 mL = 0.304 mmoles B.

    ...............Base + H^+ ==> Acid
    initial mmols..0.304..0.......0.196
    change mmols...-1.....1.........1
    final mmols....0.204..0........1.196
    Substitute into the HH equation. I obtain something like 6.3 or so but you need to confirm that. Check my work.

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