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Order of acidic character is H3PO4 [pKa~ 2.1] .
It is based on one important observation, here the number of doubly bonded oxygens that is oxygens which can take part in resonance stabilisation of negative charge of conjugate base remains same as we move across series of related oxyacids of phosphorous reverse to the trend in case of series of related oxyacids of other elements like HNO2 [HO-N=O], HNO3 [HO-N(O)=O] or HClO [HO-C]l, HClO2 [HO-Cl=O] , HClO3 [HO-Cl(=O)2], HClO4 [HO-Cl(=O)2] or H2SO3 [(HO)2-S(=O)], H2SO4 [(HO)2-S(=O)2]. For series of related oxyacids of a given element, acidic strength is inversely proportional to formal charge on oxygen in conjugate base (number of negative charge (say X) being delocalised on “Y” number of oxygens). If formal charge is high , then anion would be less delocalised (less stabilised) due to greater charge density and hence would have larger attraction for proton (H+) making them more basic and hence corresponding acid is less acidic..
If formal charge is same as in the case of conjugate bases of H3PO2, H3PO3, H3PO4 that is (H2PO2)-, (H2PO3)- and (H2PO4)- then as the number of directly bonded (to central atom) -OH increases , stability of conjugate base decreases due to participation of lone pair of -OH in “cross-conjugation” which leads to extra resonating structures where delocalisation of negative charge of conjugate base becomes somewhat less effective. Hence H3PO2 is most acidic as cross conjugation is not possible in (H2PO2)- whereas H3PO4 is less acidic due to greater cross-conjugation owing to presence of two -OH groups in conjugate base (H2PO4)-.
Note- However, these extra resonating structures are highly charged (having greater charge separation) and hence very less stable so their contribution to resonance is negligible. Thereby (H2PO2)-, (H2PO3)- and (H2PO4)- all are almost equally stable and hence H3PO2, H3PO3, H3PO4 have almost similar acidic character- H3PO4 [Ka ~ 0.0075]
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