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Worked Example

The speciation of carbon in the oceans has important effects on marine life. Below is a speciation plot showing how pH affects the distribution of HA(aq), a weak acid, and A-(aq), the conjugate base.

  1. Estimate the pKa and Ka of the weak acid.
  2. At what pH is [HA]/[A-]=100/1?
  3. At what pH does [HA]/[A-]=1/1000?
Click here to show answer

  1. Remember that, when [HA]=[A-], pH=pKa. For this speciation plot, the pH when the two curves intersect is about 7.4. Therefore, pKa is 7.4 and Ka = 10-7.4> = 4.0×10-8.

  2. Recall that the equation

    describes pH-dependent speciation of a weak acid and its conjugate base. From this equation, we can determine pH when [HA]/[A-]=100/1, using the Ka determined in the question above:

    When [H3O+]=4.0×10-6 mol L-1,

  3. When [H3O+]=4.0×10-11 mol L-1,


Your Turn

Although carbonate species are changing ocean pH, boric acid (H3BO3) and aquated borate ions (B(OH)4-) also play a key role in determining ocean pH. The Ka of boric acid is 7.24×10-10. Based on this information, determine the H3BO3/ B(OH)4- ratio at the following pH values:

  1. pH 4.00
  2. pH 7.00
  3. pH 10.00
Click here to show answer
  1. At pH 4, [H3O+] = 10-4 = 1.0×10-4 mol L-1.

    The H3BO3/B(OH)4- ratio is,

  2. At pH 7, [H3O+] = 10-7 = 1.0×10-7 mol L-1.

    The H3BO3/B(OH)4- ratio is,

  3. At pH 10, [H3O+] = 10-10 = 1.0×10-10 mol L-1.

    The H3BO3/B(OH)4- ratio is,

Acids and Bases: Speciation and pH
page 9/12

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