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Kw for ph of a strong base.

I don't get why you can use the Kw equation to calculate ph of a base? If Kw is for water then how does the same value also work for bases?
how do you know the H+ concentration isn't different to it would be in water? Also if you can use it for a base why can't you use it for an acid?

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Reply 1

KaylaB

Original post by maths_4_life

As in water, the concentration of OH- ions are the same as H+ ions, which is why when calculating pH for weak acids you can use [H+]^2

Also, a quicker way I find to calculate the pH of a strong base is to find the pOH which is basically just log of whatever concentration of the base you are given, then you minus that answer away from 14
As pH + pOH = 14

Reply 2

Pigster

In an acidic solution, there are some OH- ions, not many, but they are there.

Likewise, in alkaline solutions, there are some H+ ions.

It turns out that if you multiply the conc of OH- by the conc of H+ at 25oC, you always get the number 1.00 x10^-14.

Go figure.

Reply 3

Pigster

Original post by KaylaB

As in water, the concentration of OH- ions are the same as H+ ions, which is why when calculating pH for weak acids you can use [H+]^2

Actually, it is [A-] that equals [H+].

That pOH calc (which I prefer and teach) only works at 25oC. If you're told the value of Kw 'cos it isn't 25, then you can't use pOH (easily).