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# OCR A: Kw explanation watch

1. Can someone tell me what the point of kW is (ionic product of water) and where we even need it in calculations

also, why is H20 cancelled oout of the Kw equation

Thank you
2. (Original post by Frank Peters)
Can someone tell me what the point of kW is (ionic product of water) and where we even need it in calculations

also, why is H20 cancelled oout of the Kw equation

Thank you
The concentration of water is cancelled out because it is effectively unchanged. The proportion of water molecules that ionise is very small, hence most water molecules remain as water molecules.

You can use Kw to work out the pH of a base solution; in other words find [H+] when no acid has been added. If you know the concentration of OH- and the Kw, you can then calculate [H+] and therefore pH.
3. kly
(Original post by TutorsChemistry)
The concentration of water is cancelled out because it is effectively unchanged. The proportion of water molecules that ionise is very small, hence most water molecules remain as water molecules.

You can use Kw to work out the pH of a base solution; in other words find [H+] when no acid has been added. If you know the concentration of OH- and the Kw, you can then calculate [H+] and therefore pH.

Thank you, Kw is the equilibrium concentration of H+ ions in a solution

Also, why do we do this calculation for finding pH of Kw:
[H+]=[OH-]
Kw = [H+][OH-]
Kw = [H+]^2

If H+ concentration is equal to OH- concentration, then why wouldnt it be
Kw = [H+] rather than Kw= [H+][OH-]
4. (Original post by Frank Peters)
kly

Thank you, Kw is the equilibrium concentration of H+ ions in a solution

Also, why do we do this calculation for finding pH of Kw:
[H+]=[OH-]
Kw = [H+][OH-]
Kw = [H+]^2

If H+ concentration is equal to OH- concentration, then why wouldnt it be
Kw = [H+] rather than Kw= [H+][OH-]
[H+] = [OH-] only for pure water.

For solutions their concentrations are no longer equal.
Solutes added to the water will cause more [H+] or more [OH-].
Because Kw is constant (at a given temp) a raise in [OH-] means [H+] decreases. If we know [OH-] and Kw we can calculate [H+] and from that we can calculate pH.

Kw is what to use when you get a question alonh the lines of ' what is the pH of a 0.1M solution of sodium hydroxide'.

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