Phosphoric acid is a triprotic acid with pKa's of 2.14, 6.86, and 12.4.The ionic form that predominates at pH 3.2 is?choose the corect answer
(a) H3PO4
(b) H2PO4^-
(c) HPO4^2-
(d) PO4^3-
I can't draw the titration curve for H3PO4 on this board but you can draw it on a piece of paper. You look at the curve and figure out what you have at certain points.
Beginning is H3PO4 and pH is about 1.5 or so for a 1 M solution of H3PO4.
First equivalence point is H2PO4^- and that occurs at pH = 1/2(pK1 + pK2) = about 4.5. So, between the beginning and the first equivalence the pH is somewhere between 1.5 or so and 4.5 and the species must be H3PO4 and H2PO4^-.
For different values of pH, it is done the same way. The pH at the second equivalence point is 1/2(pK2 + pK3) and at the third is approximately sqrt (KwK3/salt) = about 13 for a 1 M concn of the salt.
The equation for each equivalence point is
H3PO4 + OH^- ==> H2O + H2PO4^-
H2PO4^- + OH^- ==>H2O + HPO4^-2
HPO4^-2 + OH^- ==>H2O + PO4^-3
To determine the ionic form that predominates at pH 3.2 for phosphoric acid (H3PO4), we need to compare the pH value with the pKa values.
In this case, we are looking for the form of phosphoric acid with a pKa that is closest to the given pH of 3.2.
The pKa values for phosphoric acid are:
pKa1 = 2.14
pKa2 = 6.86
pKa3 = 12.4
We can see that at pH 3.2, it is closer to pKa1 than the other pKa values. Therefore, the most significant ionic form at pH 3.2 is the one associated with pKa1.
The acid H3PO4 loses one proton to become H2PO4^- (di-hydrogen phosphate ion) after the first ionization step. So, the correct answer is (b) H2PO4^-.
To determine the ionic form that predominates at pH 3.2 for phosphoric acid (H3PO4), we need to consider the pKa values and the pH.
The pKa values for phosphoric acid are 2.14, 6.86, and 12.4. These values represent the acidity constant for each deprotonation step.
At pH 3.2, only the first proton (pKa 2.14) will be mostly dissociated. This means that the ionic form with one proton removed will predominate.
The correct answer is (b) H2PO4^-.