Calculting pH Of Weak Acids

Question 1

What is the strength of an acid

Answer 1

The extent of ionisation

Question 2

What is the concentration of an acid

Answer 2

The amount in moles of acid in a cubic decimetre

Question 3

What is the acid dissociation constant (Ka)

Answer 3

The equilibrium constant(acid dissociation constant) for the ionisation of a weak acid

Question 4

Ka general equation

Answer 4

[H+(aq)][A-(aq)]/[HA(aq)] With A- is the ion produced when the acid ionises

Question 5

What does a higher Ka mean

Answer 5

More likely it is to dissociate so stronger acid

Question 6

What is pKa

Answer 6

Log of acid constant, pKa=pH

Question 7

How to get pKa from ka

Answer 7

PKa= -logKa

Question 8

How to get ka from pka

Answer 8

ka= 10^-pka

Question 9

Two assumptions made when using ka

Answer 9

-[H+]=[A-], in equation all H+ coming from the dissociation of acid but also dissociation of water so [H+]>[A-] due to water contributing to [H+]
-[HA]start= [HA]equilibrium, second is that concentration of acid doesn’t change as equilibrium lies so far to the left it can be considered equal as only tiny amount of HA dissociates

Question 10

Process of working out pH of weak acid from its concentration and ka

Answer 10

ka= [H+]^2/[HA]
[H+]= square root of ka[HA]
Then -log[H+]