chemistry - pKa/pKin values

rae

What are they?! What do they tell you? Or do you just use them as part of your pH equation? So confused!!

Reply 1

Ralfskini

10

rae

What are they?! What do they tell you? Or do you just use them as part of your pH equation? So confused!!

A pKa value is used to determine the pH of a solution containing a weak acid. Recall that Kp is defined for a general reaction:

nA+mB --> pC + qD as [D]^q[C]^p/[A]^n^m, where all concentrations are at equilibrium.

In a similar way, the dissosciation of an acid can be represented by a similar equation. However because water is in excess, its concentration can be omitted.

So for the monotprotic dissosciation of an acid, HA:

pKa is given by [H+][A-]/[HA].

This means that given a value of pKa, and knowing the initial concentration of the solution, it is possible to calculate [H+] (by using the fact that [H+]=[A-] at equilibrium) and therefore the pH of the solution.

The same applies for an indicator, HIn. Water is also omitted and the dissosciation is as before.

Reply 2

rae

OP

thanks

one more q: for buffer solutions, why is it that pH is maintained when strong acids or alkalis are added?

Reply 3

Ralfskini

10

rae

thanks

one more q: for buffer solutions, why is it that pH is maintained when strong acids or alkalis are added?

Because buffers are able to resist small changes in the concentration of H+ ions in solutions.

A buffer consists of a weak acid and a salt of the weak acid. The weak acid will only be partially dissosciated, and the equilibrium will be represented by

HA <----> H+ + A-

so you can see that if the [OH-] were to increase, then this would react with the H+ ions and move the equilibrium to the right (to replace the H+ ions).

This means that [H+] will not have changed.

The addition of the salt gives an excess of A- ions in solution. So an increase in [H+] will simply cause the equilbrium to move to the left.

Reply 4

rae

OP

what im getting at is surely the pH will be maintained with additions of weak acids/bases, which can be added to a greater degree than strong ones?

Reply 5

Ralfskini

10

rae

what im getting at is surely the pH will be maintained with additions of weak acids/bases, which can be added to a greater degree than strong ones?

Yes I would have thougt so. A buffer will be able to resist small changes in pH which means that it can 'cope' with the addition of a lot of weak acid/base or a smaller volume of strong acid/base since stronger acids and bases will supply more H+ or OH- ions.

Reply 6

androidkiller

Ralfskini

A pKa value is used to determine the pH of a solution containing a weak acid. Recall that Kp is defined for a general reaction:

nA+mB --> pC + qD as [D]^q[C]^p/[A]^n^m, where all concentrations are at equilibrium.

In a similar way, the dissosciation of an acid can be represented by a similar equation. However because water is in excess, its concentration can be omitted.

So for the monotprotic dissosciation of an acid, HA:

pKa is given by [H+][A-]/[HA].

This means that given a value of pKa, and knowing the initial concentration of the solution, it is possible to calculate [H+] (by using the fact that [H+]=[A-] at equilibrium) and therefore the pH of the solution.

The same applies for an indicator, HIn. Water is also omitted and the dissosciation is as before.

Actually, Ka = [H+][A-]/[HA]. To get p of something you have to work out minus log to the base 10 of it, just like pH is -log10 [H+]. pKIn is the pH at which the indicator changes colour.

Also, you can get given the Ka value and told to work out the pH. You awould have to use the equation Ka = [H+][A-]/[HA] , using [HA] = initial concentration of acid as weak acids have little dissocaition and for an acid on its own [H+] = [A-].

Reply 7

Ralfskini

10

pKa isnt on my syllabus and I have never heard of it. I assumed it meant the same thing a Ka.

Reply 8

rae

OP

androidkiller

Actually, Ka = [H+][A-]/[HA]. To get p of something you have to work out minus log to the base 10 of it, just like pH is -log10 [H+]. pKIn is the pH at which the indicator changes colour.

Also, you can get given the Ka value and told to work out the pH. You awould have to use the equation Ka = [H+][A-]/[HA] , using [HA] = initial concentration of acid as weak acids have little dissocaition and for an acid on its own [H+] = [A-].