# Chemistry/pH- Weak Acid

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in response to Chemistry/pH- Weak Acid.
but the rule of 5%?
0,0100/(1.00x10^-4) =100%
The aproximation you do is not valid...or?
Ka= (1,0*10^(-4))^2/(x-1*10^(-4) )=1,00*10^(-6)....I got c=0,0101M
:-(

• Chemistry/pH- Weak Acid -

The Rule of 5% may be reasonable but arbitrary. I have not seen it in any textbook I use. If the approximate solution is not satisfactory solve the quadratic equation:
Ka = x^2 / (c-x)
with he proper values for the Ka and c (the overall acid concentration). That is tedious solution but sometimes necessary. One of the two roots is extraneous and must be discarded. The acceptable root should be somewhat close to the approximate solution.

• Chemistry/pH- Weak Acid -

Qualitative analsy

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