Strong acids and bases ionize completely in aqueous solutions. Weak acids and bases ionize only partially in aqueous solutions. An acid-base reaction is shown below. A titration was done by adding a base to a known concentration and fixed volume of an acid. The reaction follows:
The dissociation of a weak acid can be represented as:
Chemists often use Henderson-Hasselbach equation to calculate the pH of solutions containing a weak acid of known pKa, provided that the other concentrations are known. Henderson-Hasselbach equation is:
This equation is also used to calculate the buffering capability of solutions. It is roughly estimated that a given buffer is most likely to be effective at pH levels between (pKa + 1) and (pKa – 1).
What are the base-to-acid ratios corresponding to the lower and upper limits of pH values which make the best buffer?
|A||10 and 1|
|B||100 and 1|
|C||0.01 and 10|
|D||0.1 and 10|
In order to have a pH value of pKa – 1, the second half of the equation with the log should be negative 1. For this the base-acid ratio should be 0.1; log 0.1 = –1. For the other criterion pKa + 1, the log value should be +1. For this, the ratio should be 10.