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The pK3 of formic acid is 3.75

A)what is the pH of a buffer in which formic acid and sodium formate have equimolar concentration?
B)what is the pH of a solution in which the sodium formate is 10M and the formic acid is 1M?

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4 answers

  1. I don't believe formic acid has a pK3. You must mean pKa.
    a. pH = pKa + log (base)/(acid)
    Since base = acid then base/acid = 1; log 1 is zero and pH = pKa.

    b. Use the Henderson-Hasselbalch equation above and substitute for base and acid then calculate pH.

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  2. Can you explain it better we JUST started this section and im having a hard time with knowing what to put where when it comes to the solutions. what is the acid and what is the base

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  3. a. pH = pKa + log(base)/(acid)
    pH = 3.75 + log(x)/(x) (the problem just says base = acid so I've called that x.
    pH = 3.75 + log(1)
    pH = 3.75 + 0
    pH = 3.75

    b.
    HCOOH = formic acid = acid = 1.0M
    HCOONa = sodium formate = base = 10M
    pH = 3.75 + log (10)/(1)
    Solve for pH You should get 4.75.
    Acids/bases are described with the Bronsted-Lowry theory as
    HA = acid
    HA + H2O ==> H3O^+ + A^-
    HA is the acid and A^- is its conjugate base.
    H2O is the base and H3O^+ is its conjugate acid.
    How do you know which is which? The acid is the one with more atoms; the base is the one with fewer H atoms. Look at HA. HA and A. HA is the acid because it has more H atoms than the A. A is the base becasuse it has fewer H atoms than HA.
    H2O/H3O^+. H3O^+ has more H atoms; it's the acid. H2O has fewer H atoms; it's the base.

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  4. Thank you :) this helped a lot!

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