Calculating pH of Strong and Weak Acids with 10^-7 M Concentration - Explained

[benzun_1999]

Can anyone 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

 

[siddharth]

Why is it necessary to take into concideration the change in conc of H^+ 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 K_w. So, can you take it from here?

 

[benzun_1999]

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

 

[siddharth]

No. You will have to solve the quadratic.