Question

Oxidation number of phosphorus in ${\text{Ba}}{\left( {{{\text{H}}_{\text{2}}}{\text{P}}{{\text{O}}_{\text{2}}}} \right)_{\text{2}}}$ is _______.
A) $- 3$
B) $+ 1$
C) $+ 3$
D) $+ 5$

Answer 1

Oxidation number is the charge acquired by the species may be positive, negative or zero by gain or loss of electrons. To solve this we must know the oxidation number of barium, hydrogen and oxygen.

Complete answer:
We are given a compound ${\text{Ba}}{\left( {{{\text{H}}_{\text{2}}}{\text{P}}{{\text{O}}_{\text{2}}}} \right)_{\text{2}}}$. In the given compound, we have to find the oxidation number of phosphorus.
We know that barium belongs to group two in the periodic table. Thus, the charge on the barium atom is $+ 2$. The charge on hydrogen atom is $+ 1$ and the charge on oxygen atom is $- 2$.

We know that the sum of all the oxidation numbers in a neutral compound is zero. Thus, the total charge on the compound is zero. Thus,
Charge on ${\text{Ba}}{\left( {{{\text{H}}_{\text{2}}}{\text{P}}{{\text{O}}_{\text{2}}}} \right)_{\text{2}}}$ $= {\text{ON of Ba}} + 4 \times \left( {{\text{ON of H}}} \right) + 2 \times \left( {{\text{ON of P}}} \right) + 4 \times \left( {{\text{ON of O}}} \right)$
Substitute zero for the charge on ${\text{Ba}}{\left( {{{\text{H}}_{\text{2}}}{\text{P}}{{\text{O}}_{\text{2}}}} \right)_{\text{2}}}$, $+ 2$ for the charge on ${\text{Ba}}$, $+ 1$ for the charge on ${\text{H}}$, x for the oxidation number of ${\text{P}}$ and $- 2$ for the charge on ${\text{O}}$. Thus,
$\Rightarrow 0 = \left( { + 2} \right) + \left( {4 \times \left( { + 1} \right)} \right) + \left( {{\text{2}} \times {\text{x}}} \right) + \left( {{\text{4}} \times \left( { - 2} \right)} \right)$
$\Rightarrow 2x = 0 - 2 - 4 + 8$
$\Rightarrow 2x = + 2$
$\Rightarrow x = + 1$
Thus, the oxidation number of phosphorus in ${\text{Ba}}{\left( {{{\text{H}}_{\text{2}}}{\text{P}}{{\text{O}}_{\text{2}}}} \right)_{\text{2}}}$ is $+ 1$.

Thus, the correct answer is option (B) $+ 1$.

Note: Rules for assigning the oxidation number to elements are as follows:
i) The oxidation number of an element in its free or uncombined state is always zero.
ii) The oxidation number of monatomic ions is the same as that of the positive or negative charge on the ion.
iii) The sum of all the oxidation numbers in a neutral compound is zero.
iv) The oxidation number of alkali metals is always $+ 1$ and the oxidation number of alkaline earth metals is always $+ 2$.
v) The oxidation number of hydrogen is always $+ 1$ and the oxidation number of oxygen is always $- 2$.