Acid Base Confusion: Solutions to Common Questions

[UchihaClan13]

Hey all!
I have a silly doubt
In acids and bases
As you guys know
The concentration of H+ ions
For a weak acid
Which does not undergo
Complete dissociation
Is given by mod H+=square root of Ka times C
Where C is the initial concentration
Of the acid
And k is the ionization constant
In the same manner
For a weak base
The concentration of OH- ions
Is given in a similar manner
And ka is replaced by kb (assuming C remains constant for both of them)
Now assume that the acid and the base both are at 25 degrees centigrade
Thus kw=10^-14
Now multiplying mod H+
And OH-
We get 10^-14=10^-7 (square root of kw) times C
And thus C comes out to be 10^-7
Here is where the contradiction arises
If the concentration of the acid is less than 10^-7
And same for the base
Dissociation does occur
And their respective ph
Can be found
So why does C appear to be only 10^-7
Why a constant
Because concentration varies as per our will
We can add more solute in a solution or decrease the amount
If we want to!
Help is much appreciated

 

[UchihaClan13]

Guys
My doubt
Got cleared
I wasn't thinking properly
Thanks anyways
:)

 

[Borek]

For a weak acid
Which does not undergo
Complete dissociation
Is given by mod H+=square root of Ka times C

This is only an approximation, which holds under specific circumstances - you can't use it outside of the applicability range.

See http://www.chembuddy.com/?left=pH-calculation&right=pH-weak-acid-base for a discussion.