Quantitative Relationships Involving Carboxylic Acids

The first case simply involves the ionization of a weak acid and is governed by the expression that defines Ka for acetic acid  

Since ionization of acetic acid gives one H for each CH3C02, the concentrations of the two ions are equal, and setting each one equal to x gives  

Thus when acetic acid is added to pure water, the ratio of acetate ion to acetic acid is 

Only 1.3% of the acetic acid has ionized. Most of it (98.7%) remains unchanged. 

Now think about what happens when the same amount of acetic acid is added to water that is buffered at pH = 7.0. Before doing the calculation, let us recognize that it is the [CH3CO2 ]/[CH3C02H] ratio in which we are interested and do a little algebraic manipulation. Since 

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Quantitative Relationships Involving Carboxylic Acids CHAPTER 20... 

Apply the Henderson-Hasselbalch equation (see "Quantitative Relationships Involving Carboxylic Acids," the box accompanying Section 19.4) to calculate the CH3NH3+/CH3NH2 ratio in water buffered at pH 7. 

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