Solving for pH with diprotic/triprotic acids

[dnt]

i know if you have an acid, say .0001 M HCl, to get the pH you simply take the -log of the acid concentration (which is the same as [H+]), but what if you have a diprotic (H2SO4) or triprotic (H3PO4) acid...do you need to multiply the acid contentration by 2 or 3 to get the new H+ concentration BEFORE plugging it into -log[H+]?

and the same goes for pOH? eg. Mg(OH)2 needs to be multiplied by 2?

 

[siddharth]

Check out these links. They should help in answering your question.

http://www.science.uwaterloo.ca/~cchieh/cact/c123/polyprot.html"
http://www.sparknotes.com/chemistry/acidsbases/phcalc/section1.html"

 

[PPonte]

Mg(OH)₂ do not completely ionise in water as do for instance NaOH or KOH.

 

[Borek]

Depending on your level of education - you may either calculate pH assuming full dissociation, or you may need to calculate dissociation equilibrium. Check out these links: general method for polyprotic acids and bases pH calculation and simplified method of pH calculation for polyprotic substances.Borek