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
:-(
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.
Qualitative analsy
In the given question, it seems like you are trying to find the concentration of a weak acid using the pH and Ka values. However, I'm not sure what you mean by the "rule of 5%." Can you please explain what you are referring to so I can help you better?
Regarding your second comment, it looks like you are trying to calculate the concentration (c) of the weak acid by using the Ka value. You have correctly set up the equation for Ka:
Ka = (1.0 x 10^(-4))^2 / (x - 1.0 x 10^(-4))
Where x is the concentration of the weak acid.
To solve for x, you would rearrange the equation:
(x - 1.0 x 10^(-4)) = (1.0 x 10^(-4))^2 / Ka
Once you calculate the value inside the parentheses on the right side, you can then add 1.0 x 10^(-4) to it to find the concentration of the weak acid (x).
Now, the approximations you made might affect the accuracy of the final result. To assess the validity of your approximation, it would be helpful to know the specific context and level of precision required for the calculations.
If you can provide more information or clarify your question further, I'll be happy to assist you.