Salts 3. a Show the reaction that represents forming a conjugate base from HP Id

Question

Salts 3. a Show the reaction that represents forming a conjugate base from HP Identify the equilibrium constant value associated with this reaction. how the reaction that repzesents forming a conjugate acid from HPO.. Identify the equilbrium constant value associated with this reaction. c. In an aqueous solution, is HPO2 more likely to act as an acid or a base? d Esxplain why one substance can have both s comjugte acd and a conjugste bate e. If you were asked whether a solution of Na,HPO, were acidic or basic, how would you reply? Why?

Explanation / Answer

1. HPO4-2 + H2O(l) ? PO4-3 + H3O+ (aq), K = [PO4-3][H+]/[HPO4-2] (since, [H3O+] = [H2O][H+])

2. HPO4-2 + H3O+ (aq) ? H2PO4-1 + H2O(l) , K = [H2PO4-1]/[HPO4-2][H+]

3. Since H3PO4 can lose 3 electrons and hence has three Ka values as :-

Ka1 - when H3PO4 converts into H2PO4-

Ka2 - when H2PO4- converts into HPO4-2

Ka3 - when HPO4-2 converts into PO4-3

The experimental pKa values of aforementioned ka values are :-

pKa1 = 2.12, pKa2 = 7.21, and pKa3 = 12.67

Hence , we can conclude that it is less favorable for phosphorus compounds to lose another proton (H+) if one or more of it has already been lost and the ion is more negatively charged. Thus , HPO4-2 is very less reluctant to lose proton (act as acid ) in aquous solution. Also we now that if ka is very less of a substance then kb will be more of its conjugate base and thus HPO4-2 can acts as good base (kb = 1.6 *10-7) relative to acid. Although its Kb and Ka values are close and very small and thus we can say that it acts as both weak acid and weak base (amphoteric in nature).

4. Suppose a substance to be of form AHx (x>1) and also it has atleast one H atom which it can donate or loose to form its conjugate base AHy where y<x and y>=1. Now Since its conJugate base still has atleast 1 H+ to loose and thus can act as both conjugate base and acid.

5. As explained above in part 3, that HPO4-2 is amphoteric in nature and thus acts as both acid and base. This is due to the fact since it has one H left and can loose it further (act as weak acid) and also can gain one or more H+ (act as base).

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