Will we be ask to include "the dissociation of water" in our pH calculation?

[Smooth Operater]

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Will we be ask to include "the dissociation of water" in our pH calculation? Does such question actually appear on exam? Thanks!

 

[Smooth Operater]

bump!!

 

[aranjuez]

Can you give an example? H+ ions are supposed to be counted regardless of source. An example would be nice. 🙂

aranjuez

 

[Smooth Operater]

This is my personal note which is taken from kaplan blue book

-the contribution of OH- and H+ ions from the dissociation of H2O can be neglected only if the concentration of the acid or base is greater than 10-7.

-For example, the pH of a 1e-8 M HCl may be 8. But a pH of 8 is in the basic range, and
HCl solution is not basic. The discrepancy arises from the fact that at low HCl conc. H+ from the dissociation of water contribute significantly to the total [H+]. Thus, the total concentration of H+ can be calculated from:

Kw = (x + 1e-8)(x) = 1.0e-14 ; where x = [H+] = [OH-] from the dissociation of water molecule

-solving for x gives x = 9.5e-8 M

so, [H+]total = 9.5e-8 + 1e-8 = 1.5e-7 M

and pH = -log (1.05 e-7) = 6.98





But, on the real DAT, would we get question that ask us to include dissociation of water?