If the ph at the half-titration point of a monoprotic weak acid is 4. 2, what is the ka of the acid?.

Finding the ka of the acid- The idea is that at the half-equivalence point, the pH of the solution will be equal to the pKa of the weak acid.
If you're titrating a weak monoprotic acid, which I'll refer to as HA, you already know that at the equivalence point, the strong base will completely neutralize the weak acid.

HA(aq)+OH−(aq)→A−(aq)+H2O(l)

Because of this, when you combine an equal number of moles of a weak acid with a strong base, all of the moles of the weak acid will be consumed, leaving you with A, the conjugate base of the weak acid.
You have now added enough moles of the strong base to the solution to neutralize half of the weak acid molecules.
The equivalent point by half refers to this location.
The ratios of the weak acid, strong base, and conjugate base are all 1:1, meaning that what you produce as the conjugate base is what you eat as the weak acid and strong base.
Half of the weak acid's moles will be consumed by the reaction, while the conjugate base is generated in an equal amount.
Consequently, when a weak acid and a strong base are combined in an equal amount.
The weak acid and its conjugate base will therefore be present in equal amounts in the solution at the 50% equivalence point, suggesting that you are now working with a buffer solution.
As you are aware, the pH of a weak acid-conjugate base buffer can be determined using the Henderson-Hasselbalch equation pH=pKa+log ([conjugate base][weak acid])

You reach the halfway point when

[HA]=[A−]

It implies that

log([HA][A−])=log(1)=0

As a result, it can be claimed that at the half-equivalence point, the pH of the solution and the pKa of the weak acid are equivalent.

pH=pKa at the midpoint of equivalency.

The pKa is determined by the weak acid's acid dissociation constant, Ka. Ka=10pKa, as shown by the formula pKa=log(Ka).

When the two points are half equal, Ka=10pH will exist.

Put in your value to learn Ka=105.67=2.1106.

To learn more about half-equivalence point visit:

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