Mistake Master
pH and pKa
Whether a molecule is holding its proton or has let it go depends on one comparison: pH versus pKa. That single rule also explains why an indicator changes color right where it does.
§1
Which form dominates.
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Comparing the solution's pH to a species' pKa tells you which form predominates. When pH < pKa, the protonated (acid, HA) form dominates; when pH > pKa, the deprotonated (conjugate base, A⁻) form dominates.
At pH = pKa, the two forms are present in equal amounts.
An indicator is itself a weak acid whose protonated and deprotonated forms have different colors. It changes color as the pH crosses its pKa, which is exactly why an indicator signals the pH range around that pKa.
§2
Applying the comparison.
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Compare pH to pKa, then name the form.
- Find the species' pKa. The pKa of the acid form.
- Compare it to the solution pH. Is the pH below, above, or equal to the pKa?
- Name the predominant form. pH < pKa → protonated (HA); pH > pKa → deprotonated (A⁻); equal at pH = pKa.
- Apply to indicators. An indicator's two colored forms predominate on either side of its pKa, so it changes color there.
§3
The pieces you'll meet.
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One comparison, one predominant form.
§4
Worked example: predominant form and indicators.
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Species. A weak acid HA has pKa = 5.
At pH 3. pH (3) < pKa (5), so the protonated HA form predominates.
At pH 7. pH (7) > pKa (5), so the deprotonated A⁻ form predominates.
Indicator. An indicator with pKa ≈ 5 would show its acid-form color below pH 5 and its base-form color above pH 5, changing color as the pH crosses 5.
§5
Mistakes that cost real points.
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"When pH is below the pKa, the deprotonated form dominates."
It is the reverse: when pH < pKa, the protonated (acid, HA) form dominates; the deprotonated (A⁻) form dominates when pH > pKa. Mixing up the direction misidentifies the predominant species.
Fix. Use: pH < pKa → protonated (HA); pH > pKa → deprotonated (A⁻).
"An indicator changes color at pH 7."
An indicator changes color around its own pKa, not at pH 7. Different indicators have different pKa values and change color at different pH ranges — that is why the right indicator is chosen to match a titration's equivalence pH.
Fix. Match an indicator's color change to its pKa, not to pH 7.
"At pH = pKa, the solution is neutral (pH 7)."
pH = pKa means the protonated and deprotonated forms are equal — it is not a statement that the pH is 7. A species with pKa = 4 has equal forms at pH 4, which is acidic, not neutral.
Fix. Read pH = pKa as equal amounts of the two forms, at whatever pH the pKa happens to be.
§6
Skill Check.
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Ten scenarios. Pick the chips that match your answer, then check. A scenario marks complete the first time every part is right. Progress saves on this device.