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Q.
For $H_3PO_3\, and\, H_3PO_4,$ the correct choice is
IIT JEEIIT JEE 2003The p-Block Elements - Part2
Solution:
$H _{3} P O _{3}$ is a dibasic acid as it has $2$ number of replaceable $OH$ groups(Dibasic) attached to phosphorous. The oxyacids of phosphorous are reducing agents. $H _{3} PO _{3}$ is a good reducing agent than $H _{3} PO _{4}$. The reducing nature of these acids is based on the no of hydrogen atoms bonded to the central metal atom. As more number of $H$ attached to phosphorous more is its reducing nature. In the case of $H _{3} P O _{3}$, there is one $P - H$ bond which is responsible for it's reducing character whereas there is no such bonding in case of $H _{3} P O _{4}$.