# 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.