Question about Ka and Kw in OCR Chemistry?
So basically say for instance the equation HA ----> H+ + A-
If we are calculating Ka then Ka = [H+]^2/[HA]
So we say that the amount of A and H formed are the same so we can just say [H+]^2
But surely different there can be different concentration of A than H or vice versa?
So in Kw=[H+][OH-] even though they are formed from H2O we dont assume that equal amounts have been formed?
I just donnnnnnnnn't understand :/
Can someone explain how we can make an assumption for one and not the other? Isn't Kw basically just Ka but for water?
If we are calculating Ka then Ka = [H+]^2/[HA]
So we say that the amount of A and H formed are the same so we can just say [H+]^2
But surely different there can be different concentration of A than H or vice versa?
So in Kw=[H+][OH-] even though they are formed from H2O we dont assume that equal amounts have been formed?
I just donnnnnnnnn't understand :/
Can someone explain how we can make an assumption for one and not the other? Isn't Kw basically just Ka but for water?
Original post by FatFace817
So basically say for instance the equation HA ----> H+ + A-
If we are calculating Ka then Ka = [H+]^2/[HA]
So we say that the amount of A and H formed are the same so we can just say [H+]^2
But surely different there can be different concentration of A than H or vice versa?
So in Kw=[H+][OH-] even though they are formed from H2O we dont assume that equal amounts have been formed?
I just donnnnnnnnn't understand :/
Can someone explain how we can make an assumption for one and not the other? Isn't Kw basically just Ka but for water?
If we are calculating Ka then Ka = [H+]^2/[HA]
So we say that the amount of A and H formed are the same so we can just say [H+]^2
But surely different there can be different concentration of A than H or vice versa?
So in Kw=[H+][OH-] even though they are formed from H2O we dont assume that equal amounts have been formed?
I just donnnnnnnnn't understand :/
Can someone explain how we can make an assumption for one and not the other? Isn't Kw basically just Ka but for water?
Moved to Chemistry
If a weak acid is monobasic (meaning it produces one H+ ion) then the amount of H+ formed will equal the amount of A- formed and so the concentration of H+ will equal A- so we can use [H+]^2 in the Ka equation.
For Kw in pure water then H2O splits equalling into H+ and OH- so [H+] = [OH-] which is 1 x 10-14 at 298K but in acidic or alkaline conditions then you will have added some H+ or OH- so [H+] does not equal [OH-].
With the Ka example we also meet this in buffer solutions which are made by adding A- to a weak acid. In this case then [H+] does not equal [A-] so [H+]^2 cannot be used in the equation and we have to calculate [H+] and [A-].
For Kw in pure water then H2O splits equalling into H+ and OH- so [H+] = [OH-] which is 1 x 10-14 at 298K but in acidic or alkaline conditions then you will have added some H+ or OH- so [H+] does not equal [OH-].
With the Ka example we also meet this in buffer solutions which are made by adding A- to a weak acid. In this case then [H+] does not equal [A-] so [H+]^2 cannot be used in the equation and we have to calculate [H+] and [A-].
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