How to calculate pH of strong and weak acids at 10^-7 M concentration

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benzun_1999
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Can anyone tell me how to calculate pH of an strong and weak acid with conc [tex]10^-^7 M[/tex] . Please just explain the method. i read a little about it. Why is it necessary to take into consideration the change in conc of [tex]H^+[/tex] from water?

-Benzun
 
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benzun_1999 said:
Why is it necessary to take into concideration the change in conc of [tex]H^+[/tex] from water?

That is because the amount of H+ which comes from water is now not negligible when compared to the H+ which dissociates from the acid.

For example, what is the pH of 10^-8 M of HCl ?

What happens is that, the dissociation of H20 is suppressed by HCl by the common ion-efect.

So if you consider the dissociation of water
H20----> H+ + OH-
let x be the conc of H+
so, the total conc of H+ is x (from water) + 10^-8 (from the strong acid HCl). And the conc of OH- is x(from water). You also know Kw. So, can you take it from here?
 
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ok understood but is there some easy and clean way to proceed in this sum without getting quadratic equation?