PH of 10^-7 M acid

1. Jan 29, 2006

benzun_1999

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

-Benzun

Last edited: Jan 29, 2006
2. Jan 29, 2006

siddharth

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?

Last edited: Jan 29, 2006
3. Jan 29, 2006

benzun_1999

ok understood but is there some easy and clean way to proceed in this sum without getting quadratic equation???

4. Jan 29, 2006

siddharth

No. You will have to solve the quadratic.