Question

Which of the following statements is true?

Select one:

a. An effective buffer has a pH = pKa of the weak acid.

b. Buffer capacity depends on concentration

c. If the pH of a buffer is higher than the pKa the acid buffer capacity is greater than the base capacity.

d. They are all true.

Answer 1

The pH of an acidic buffer is given by Henderson-Hasselbach equation as:

where the salt is the salt of the weak acid and pKa is that of the acid in the buffer. Since all other parameters apart from the ratio of salt and acid in the buffer is constant, the pH of a buffer is determined by the ratio of concentrations of acid and its salt. Now, if the ratio is greater than or lesser than one, the equilbirium established by the conjugate base of the acid will not be uniform under addition of acid or base. This will result in a lower quantity of strong acid or base addition that will change the pH of the solution.

Buffer capacity is defined as the quantity of strong acid/base required to be added to the given buffer to effect a pH change of +/- 1 unit. Now if the ratio of salt to acid is less than one, greater quantity of the acid tends to ionize, giving a lower pH and ratio greater than one will give a higher pH in the buffer. Internally this slight imbalance will result in a metastable equilibrium between the conjugate base of the acid and the acid, decreasing the efficiency of the buffer. So, if the ratio of salt to acid is exactly 1, the buffer will possess maximum buffer capacity or greatest efficiency.

Though the pH of a buffer is unaffected by concentration of the salt and acid and is affected only by their mole fractions, the buffer capacity is dependent on the buffer's concentration as greater quantities of salt and acid/bse will mean greater tolerance to the addition of strong acid/base and hence a better buffer capacity.

In a nutshell, the statements that are true among the given are statements A and B.