# pKa = pH for strong acid — strong base?

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The reason is that strong acids have $$\mathrm{p}K_\mathrm{a}$$ values that are poorly known. These $$\mathrm{p}K_\mathrm{a}$$s cannot be determined directly, because the exact concentration of the ions is difficult to know with precision: the electrodes do not react with the concentration of $$\ce{H^+}$$ or $$\ce{H3O^+}$$ ions. They react with the activity of $$\ce{H+}$$ or $$\ce{H3O+}$$, which may be quite different from the concentration in concentrated solutions.

So, the $$\mathrm{p}K_\mathrm{a}$$ values of strong acids can only be determined indirectly, for example by extrapolation of values obtained in organic solvents, and mixtures of organic solvents plus water. This extrapolation is never precise enough.

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### vgupt

Updated on February 01, 2023

• vgupt 19 minutes

I have learnt that for a weak acid — strong base titration, $$\mathrm{p}K_\mathrm{a} = \mathrm{pH}$$ at the half equivalence point.

However, the same conclusion is not drawn when discussing strong acid — strong base titrations. Why does the above only hold for weak acid — strong base and not strong acid — strong base?

Similarly, why is $$\mathrm{pOH} = \mathrm{p}K_\mathrm{b}$$ at half-equivalence point only true for strong acid weak base and not for strong acid strong base?

What do you know about buffers?
I know that a buffer solution is one which resists changes to pH when small amounts of acid/alkali are added and I know the two ways in which a buffer solution can be formed. I also know that a buffer is most effective when the concentration of the ions is very high so that additional ions added (H+ or OH-) won't have a large relative impact...is that useful?
How about pKa = pH vs buffers then and why strong acid won't work?
No. The pH cannot be determined in a concentrated solution. Or, if you prefer : the measured pH values has no relation with the concentration of the ion $\ce{H^+}$.