Question

Which is the most prevalent species in a 0.25M aq solution of phosphoric acid, H₃PO₄?

Ka1 = 7.5x10^-3  , Ka2 = 6.2x10^-8 , Ka3 = 4.2x10^-13

a) H₃PO₄

b) H2PO₄^-

c) HPO₄^2-

d) H₃O⁺

The answer is A, can you explain why?

Answer 1

note that K is the ability to dissociate, if K vlaues are low, then this will NOT dissociate

all Ka values are pretty low, i.e. 10^-3

so expect low dissociation

so...

H3PO4 <-> H2PO4- + H+ (pretty low, since Ka is low)

H2PO4- <--> H+ + HPO4- (note that the marked H2PO4- is already too low, since it has very low dissociation in the first reaction... meaning that HPO4- is even lower due to the KA2 value)

therefore

Ka3 species are very very low

the most presence:

H3PO > H+ > H2PO4- > HPO4-2 > PO4-3

so

choose H3PO4, the molecular acid