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pKa is the negative logarithm of Ka: the smaller it is the stronger the acid, each unit is a factor of ten, and comparing pKa with pH says whether an acid is mostly protonated or deprotonated.
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You will say which acid is stronger from pKa, turn a pKa gap into a Ka ratio, convert Ka to pKa, rank acids, and decide an acid's form at a given pH.
An acid is a proton donor and its Ka measures how far its ionisation goes. Organic chemistry needs that idea across a vast range, from acids that give up a proton completely to C–H bonds that almost never do.
pKa $= -\log_{10} K_a$. The conjugate base is what is left when an acid has lost its proton. An acid is protonated when it holds its proton and deprotonated when it has lost it.
Because pKa is the negative logarithm of Ka, the scale runs backwards and compresses: a Ka of $10^{-5}$ is a pKa of 5, a Ka of $10^{-16}$ a pKa of 16.
Approximate values across organic chemistry:
| Acid | pKa |
|---|---|
| hydrochloric acid, HCl | −7 |
| ethanoic acid, CH3COOH | 5 |
| phenol, C6H5OH | 10 |
| water, ethanol | 16 |
| propanone, C–H next to C=O | 20 |
| ethyne, HC≡CH | 25 |
| ammonia, NH3 | 38 |
| ethene | 44 |
| ethane | 50 |
Ethanoic acid (5) is $10^{11}$ times stronger than ethanol (16): same O–H bond, vastly different acidity. The next three lessons explain differences like this one — resonance, induction and hybridization.
Another way: steps
To compare two acids:
pKa is a property of the acid; pH is a property of the solution. Put them side by side and they say which form of the acid is present. When the pH is below the pKa, the solution has more protons than the acid can resist, and the acid is mostly protonated; above the pKa, mostly deprotonated. At pH = pKa the two forms are equal. Each unit of difference is a factor of ten: aspirin (pKa 3.5) in the stomach (pH 1.5) is about 100 to 1 protonated, which is why drug chemists care where in the body a molecule's pKa sits.
A higher pKa means a stronger acid. Lower pKa is stronger.
A difference of two pKa units is twice as strong. It is a hundred times.
pKa and pH are the same thing. One describes the acid, the other the solution.
Every acid gives up its proton in water. It depends on the pH.
Phenol's pKa is about 10; ethanol's about 16.
Phenol's is smaller.
The gap is 6 units.
Subtract the pKa values.
So phenol's Ka is $10^6$, a million, times larger: phenol is the stronger acid.
A factor of ten per unit.
Which is the stronger acid?
Ethyne, with the smaller pKa.
By what factor?
Its Ka is $10^{13}$ times larger than ammonia's.
phenol, C6H5OH, has a pKa of about $10$; ethanol, CH3CH2OH, about $16$. Which is the stronger acid?
propanone has a pKa of about $20$ and ethyne about $25$. How many times larger is the Ka of propanone?
Answer:
A carboxylic acid derivative has $K_a = 1 \times 10^{-9}$. What is its pKa?
Answer:
Rank these from strongest acid to weakest, using their approximate pKa values: ethanol (16), ethanoic acid (5), ethyne (25), hydrochloric acid (−7), phenol (10).
Number the steps in order (write the number in the box):
Aspirin is a carboxylic acid with a pKa of about 3.5. In the stomach, where the pH is about 1.5, is it mostly in its protonated (neutral) form or its deprotonated (charged) form?
Lesson test: one question per skill, one attempt each, no hints. Your answers are checked when you submit.
An acid has a pKa of $12$. A second acid has a Ka $10^{3}$ times larger. What is the second acid's pKa?
The second acid's pKa is a.
You can read pKa as a comparison of acid strength. Tell someone why a pKa gap of 2 means a hundredfold difference. Next: why ethanoic acid is so much stronger than ethanol — resonance.
8. Your turn: ethyne (25) against ammonia (38), step 3