Why is pKa a more convenient measure of acid strength?Ka varies over many powers of 10.
It is more convenient to talk about acid strength using the logarithm of Ka because this makes the range of values smaller (-50 to +50).
Most acids are weak, and
Ka for these is a multiple of negative powers of ten.
Thus for most acids, it is more convenient describe acid strength using the negative of the logarithm of Ka because comparison of positive numbers is easier.
Therefore
pKa is
defined similarly to pH:
Ka and p
Ka for several acids are given in the table below. Two
Ka entries have below them calculator keystrokes that give the corresponding p
Ka.
Show Calculator | Acid | Ka | pKa |
| HCl | 1.0 × 107 | -7.00 |
| HNO3 | 4.0 × 101 | -1.60 |
| HF | 5.6 × 10–4 | 3.25 |
| HOCl | 3.0 × 10–8
-log(3.0× shift log(-8))
BC Calculator:
3.0E-8 log +/- | 7.52
shift log(-7.52) |
| HCN | 3.9 × 10–10
-log(3.9× shift log(-10))
BC Calculator:
3.9E-10 log +/- | 9.41
shift log (-9.41) |
Note, for weak acids, the relationship between first digit in pKa
and the magnitude of the exponent in Ka. |
What does comparing pKa values tell us?The data in the table show that
pKa for strong acids is negative
pKa is more negative for stronger acids
For weak acid-base conjugate pairs
pKa is a positive number
the acid with the smaller pKa is stronger
When comparing solutions of equal concentration,
the solution of the acid with smaller pKa has the lower pH.