simple chem pH question

[jabongga]

Whats the pH of 5M HCl?


Since its a strong acid, [H+] should be 5M.
pH = -log[H+]
pH = -log[5]
pH = -(0.699)
pH = -0.699


Can you have a negative pH? All the pH scales ive seen are only from 0 to 14. Is there something im missing

 

[zzzz]

Can you have a negative pH?

you sure can.

 

[jabongga]

just got this off a site:

"The pH scale is a measure of the hydrogen ion concentration. It spans from 0 to 14 with the middle point (pH 7) being neutral (neither acidic or basic).
Any pH number greater than 7 is considered a base and any pH number less than 7 is considered an acid. 0 is the strongest acid and 14 is the strongest base."
http://lrs.ed.uiuc.edu/students/erlinger/water/background/ph.html



EDIT: ok read something from another site:
http://newton.dep.anl.gov/askasci/chem99/chem99230.htm

Seems to say that negative pH values are OK. Why do books seem to imply otherwise though?

Similarly can you have a base with a pH > 14?

 

[dionx]

yeah u can have bases with pH greater than 14.

but when doing advanvced equilibria problems, pH is used but not as often as pKa or pKb. Ka and Kb can tell more things about a reaction than H+ concentration can.

 

[jabongga]

so if thats the case, why on earth do they imply that pH only ranges from 0 to 14? Those are arbitrary numbers, why not -8 to 77?

 

[dionx]

first, the constant for an weak acid is:

k = [H+] [A-] / [HA]

or in general, its k = [products] / [reactants]

when doing pH, most ppl take pH of weak acids or a general reaction that is not the dissociation.

u are not allowed to take the pH of a strong acid, coz if all if the acid dissassociated, there would be 0 concentration of reactants left, and dividing by zero is undefined here, therefore you cant take negative log of that anyways

 

[Scarpozzi]

It's all about scales here. They want a standard measurement... Who's they? The people that are already too dead to ask why the heck they made it that way... Just deal with it. 😛