Chemistry · General chemistry II · Worked example
Find the pH of a buffer
Find the pH of a buffer that is 0.100 M acetic acid and 0.150 M sodium acetate. For acetic acid, Ka = 1.8 × 10⁻⁵.
Find pKa
pKa = −log Ka.
Apply Henderson–Hasselbalch
Acetate is the base and acetic acid the acid.
Interpret
There is more base than acid, so the pH sits a little above pKa. The shortcut is safe here: the acetate already present keeps the acid’s own ionization tiny.
Result
pH = 4.92.
Your turn
What ratio of acetate to acetic acid gives a buffer with pH 5.00?
Show the answer and explanation
About 1.8 to 1.
log([A⁻]/[HA]) = 5.00 − 4.74 = 0.26, so [A⁻]/[HA] = 10^0.26 = 1.8.
Keep exploring
In Buffers & titration curves, make the two amounts equal. The pH becomes 4.74, the pKa, whatever the common amount.
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