Question

The oxidation number of arsenic atom in ${{H}_{3}}AS{{O}_{4}}$ is.
A) $\left( -1 \right)$
B) $-3$
C) $+3$
D) $+5$

Answer 1

Hint : Oxidation number is defined as the number of electrons that an atom or elements has lost or gained from the free element to its natural. Therefore oxidation number refers to the effective charge associated with an element; it may be positive or negative.

Complete Step By Step Solution
We have to calculate the oxidation of arsenic in ${{H}_{3}}AS{{O}_{4}}$ .
The sum of the oxidation number of all the atoms in the formula of the compound is always taken zero.
Oxidation number of hydrogen = $+1$
Oxidation number of oxygen = $-2$
Let the oxidation number of AS = $x$
The sum of all oxidation no = $0$
$1\times 3+x+4\left( -2 \right)=0$
$3+x-8=0$
$x-5=0$
$x=5$
Oxidation number of Ar is $+5$
So, option (D) is correct.

Note
We have to remember how to calculate the oxidation number of an atom in a compound. Transition metals show variable oxidation states. The oxidation number of molecules like ${{O}_{2,}}{{H}_{2,}}{{N}_{2}}$ etc. Is taken as zero. The sum of oxidation number of coordination compound equal to charge present on it.