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Calculation of Molar Ratios of Conjugate Base to Weak Acid from pH For a weak acid with a pKa of 6.0, calculate the ratio of conjugate base to acid at a pH of 5.0.

Choice of Weak Acid for a Buffer Which of these com-pounds would be the best buffer at pH 5.0: formic acid (pKa 5 3.8), acetic acid (pKa 5 4.76), or ethylamine (pKa 5 9.0)? Briefly justify your answer.

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Explanation:

According to the Handerson equation,  

          pH = [tex]pK_{a} + log \frac{\text{salt}}{\text{acid}}[/tex]

or,      pH = [tex]pK_{a} + log \frac{\text{conjugate base}}{\text{acid}}[/tex]

Putting the given values into the above equation as follows.

     pH = [tex]pK_{a} + log \frac{\text{conjugate base}}{\text{acid}}[/tex]

       5.0 = 6.0 + log \frac{\text{conjugate base}}{\text{acid}}[/tex]

      [tex]log \frac{\text{conjugate base}}{\text{acid}}[/tex] = -1.0

or,      [tex]\frac{\text{conjugate base}}{\text{acid}} = 10^{-1.0}[/tex]

                            = 0.1

Therefore, we can conclude that molar ratios of conjugate base to weak acid for given solution is 0.1.