A) \[{{H}_{3}}P{{O}_{3}}>{{H}_{3}}P{{O}_{4}}>{{H}_{3}}P{{O}_{2}}\]
B) \[{{H}_{3}}P{{O}_{4}}>{{H}_{3}}P{{O}_{3}}>{{H}_{3}}P{{O}_{2}}\]
C) \[{{H}_{3}}P{{O}_{4}}>{{H}_{3}}P{{O}_{2}}>{{H}_{3}}P{{O}_{3}}\]
D) \[{{H}_{3}}P{{O}_{4}}\approx {{H}_{3}}P{{O}_{3}}\approx {{H}_{3}}P{{O}_{2}}\]
Correct Answer: D
Solution :
There is very little difference in acid strength in the series \[{{H}_{3}}P{{O}_{4}},{{H}_{3}}P{{O}_{3}}\]and \[{{H}_{3}}P{{O}_{2}}\] because the hydrogen in these acids are not all bonded to phosphorus. In the above three acids although the number of-OH groups (ionisable hydrogen increases, yet the acidity does not increase very much. This is due to the fact that the number of unprotonated oxygen, responsible for the enhancement of acidity due to inductive effect, remains the same with the result dissociation constant also remains nearly same.
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