Question

How many grams of dry NH4Cl need to be added to 2.40 L of a 0.100 M solution of ammonia, NH3, to prepare a buffer solution that has a pH of 8.52? Kb for ammonia is 1.8×10−5.

Just as pH is the negative logarithm of [H3O+], pKa is the negative logarithm of Ka,

pKa=−logKa

The Henderson-Hasselbalch equation is used to calculate the pH of buffer solutions:

pH=pKa+log[base]/[acid]

Notice that the pH of a buffer has a value close to the pKaof the acid, differing only by the logarithm of the concentration ratio [base]/[acid]. The Henderson-Hasselbalch equation in terms of pOH and pKb is similar.

pOH=pKb+log[acid]/[base]

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