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Butyric acid (HC4H7O2) is a weak acid with the stench of rancid butter

1.) write the equation for the dissociation of HC4H7O2 in water

2.)the pH of .5 M HC4H7O2 is 2.56 calculate Ka of butyric acid

3.) Write the equation for the hydrolysis of C4H7O2-

4.) Calculate Kb for C4H7O2-

5.) Calculate the pH of a buffer solution in which the concentration of butyri acid= .36 M and the concentration of C4H7O2- is .31 M.

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1 answer
  1. 1.
    HC4H7O2 + H2O ==> H3O^+ + C4H7O2^-

    2.
    pH = -log(H3O^+)
    -2.56 = log(H3O^+)
    (H3O^+) = about 2.8E-3 but you need to do it more accurately. Let HB stand for butyric acid, then
    .......HB + H2O ==> H3O^+ B^-
    I.....0.5M...........0.....0
    C......-x...........x.......x
    E.....0.5-x.........x.......x

    Ka = (H3O^+)(B^-)/(HB)
    You know (H3O^+) = about 2.8E-3, that's also B^-. Substitute into Ka expression and solve for Ka.

    3.
    C4H7O2^- + H2O ==> H3O^+ + HC4H7O2

    4.
    Kb for B^- = (Kw/Ka for HB)

    5. Use the Henderson-Hasselbalch equation.

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