Question

Which of the following oxide is most acidic?
A. $A{s_2}{O_3}$
B. ${P_2}{O_5}$
C. $S{b_2}{O_3}$
D. $B{i_2}{O_3}$

Answer 1

To solve this we need to understand the concept of acid oxides acidic oxide is an oxide that either produces an acidic solution upon addition to water, or acts as an acceptor of hydroxide ions effectively functioning as a lewis acid. Higher oxidation state implies higher amount of partial positive charge on the concerned element. And according to Lewis acids and bases theory, acidity depends on the ability to accept an electron pair which in turn is dependent on the positive charge on the element.

Complete Step By Step Answer:
As the oxidation state of the central atom becomes larger, the acidity of the molecule increases. An atom becomes more electronegative as the oxidation number increases. This then causes the oxygen atoms to become more electronegative. The $O - H$ bond then becomes more polar and therefore more acidic.
Generally higher oxidation state means more bonded oxygens, thus the formation of more acidic $OH$ groups during reaction of oxide with water. When the acid is formed, higher oxidation state is linked to higher electron density shift towards the central atom. The electropositive character of the oxide's central atom will determine whether the oxide will be acidic or basic. The more electropositive the central atom the more basic the oxide. The more electronegative the central atom, the more acidic the oxide.
It is very clear that ${P_2}{O_5}$ is the only compound with five oxygen atoms, $P$ is going to be the highest oxidation state.
So, the correct answer is (B) ${P_2}{O_5}$ .

Note:
Less electronegative elements tend to form basic oxides such as sodium oxide and magnesium oxide, whereas electropositive elements tend to produce acidic oxides as seen with carbon dioxide and phosphorus pentoxide. Some oxides like aluminum oxides are amphoteric. Acidic oxides are of environmental concern.