You are Here: Home

# pH of water - ionisation product of water question watch

1. Hi guys,

Just wondering whether anyone could explain this to me... my textbook says:

In pure water [H+(aq)] = [OH-(aq)] which I get, it's neither acidic or alkaline

Kw = 1x10^-14 mol^2dm^-6 which I get because it stated that at 298K Kw = [H+][OH-] so Kw is usually about 1x10^-14 mol^2dm^-6 at 298K

I then says (and this is the bit I don't understand) Kw = [H+(aq)]^2
Why is this so and where did it come from?

Thanks in advance for any help
2. (Original post by Blake Jones)
Hi guys,

Just wondering whether anyone could explain this to me... my textbook says:

In pure water [H+(aq)] = [OH-(aq)] which I get, it's neither acidic or alkaline

Kw = 1x10^-14 mol^2dm^-6 which I get because it stated that at 298K Kw = [H+][OH-] so Kw is usually about 1x10^-14 mol^2dm^-6 at 298K

I then says (and this is the bit I don't understand) Kw = [H+(aq)]^2
Why is this so and where did it come from?

Thanks in advance for any help
In pure water there is one H+ for every OH- ion, so [H+] = [OH-]
thus: Kw = [H+]^2
3. (Original post by Blake Jones)
Hi guys,

Just wondering whether anyone could explain this to me... my textbook says:

In pure water [H+(aq)] = [OH-(aq)] which I get, it's neither acidic or alkaline

Kw = 1x10^-14 mol^2dm^-6 which I get because it stated that at 298K Kw = [H+][OH-] so Kw is usually about 1x10^-14 mol^2dm^-6 at 298K

I then says (and this is the bit I don't understand) Kw = [H+(aq)]^2
Why is this so and where did it come from?

Thanks in advance for any help
As if [H+] = [OH-]
And Kw = [H+][OH-]
Then multiplying by [OH-] would be the same as multiplying by [H+]

Thus Kw = [H+]2

Does that make sense now?
4. (Original post by Qinge)
In pure water there is one H+ for every OH- ion, so [H+] = [OH-]
thus: Kw = [H+]^2
Oh! Because there are 2H atoms effectively so the concentration is double so then it goes to squared as it's a concentration, got it! Thanks mate!
5. (Original post by KaylaB)
As if [H+] = [OH-]
And Kw = [H+][OH-]
Then multiplying by [OH-] would be the same as multiplying by [H+]

Thus Kw = [H+]2

Does that make sense now?
Yeah I get that now, thank you so much you guys! Don't know why I didn't see that before! XD
6. (Original post by Blake Jones)
Yeah I get that now, thank you so much you guys! Don't know why I didn't see that before! XD
No problem!
7. (Original post by Blake Jones)
Oh! Because there are 2H atoms effectively so the concentration is double so then it goes to squared as it's a concentration, got it! Thanks mate!
That makes it look like you didn't get it.

If you had some water at 298 K, the pH would be 7.00

[H+] = 10-pH = 10-7.00 = 0.0000007 mol dm-3

The only way you can make H+ ions from water also makes OH- ions. So the concentration of OH- ions must also = 10-7.00. Both of them have that concentration, neither have double that value.

Kw = [H+] x [OH-] = 10-7.00 x 10-7.00 = [H+]2

### Related university courses

TSR Support Team

We have a brilliant team of more than 60 Support Team members looking after discussions on The Student Room, helping to make it a fun, safe and useful place to hang out.

This forum is supported by:
Updated: January 5, 2017
Today on TSR

### Edexcel C2 Core Unofficial Markscheme!

Find out how you've done here

### Everything you need to know for GCSE maths

Poll

The Student Room, Get Revising and Marked by Teachers are trading names of The Student Room Group Ltd.

Register Number: 04666380 (England and Wales), VAT No. 806 8067 22 Registered Office: International House, Queens Road, Brighton, BN1 3XE