Question

oxidation number of $P$ in $K{H_2}P{O_3}$ is: $+ 5$
A. $- 1$
B. $- 3$
C. $+5$
D. $+ 3$

Answer 1

The oxidation number of an element can be determined by using the oxidation number of other elements and the charge neutrality principle.

Complete step by step answer:
We use the oxidation number of an element to denote the oxidation state for the same. We can determine the oxidation number by following some rules which can be listed as follows:
1. When the element is in free-state, we assign an oxidation number of zero to it. For example, in hydrogen gas molecules, the oxidation number assigned to hydrogen is zero.
2. For mono-atomic ions, the oxidation number can simply be assigned equal to the charge of the ion. For example, in chloride ions, $C{l^ - }$ , the oxidation number assigned to chlorine is $- 1$.
3. We assign an oxidation number of $+ 1$ to hydrogen. However, metal hydrides are exceptions, and hydrogen gets assigned an oxidation number of $- 1$ in these.
4. We assign an oxidation number of $- 2$ to oxygen in most of the compounds. However there are some exceptions to this: peroxides and superoxides. We assign an oxidation number of $- 1$ to oxygen in peroxides and an oxidation number of $- \left( {1/2} \right)$ to oxygen in superoxides. Another exception being the compounds of oxygen with fluorine where it gets assigned a positive oxidation number.
5. Among halogens, an oxidation number of $- 1$ is assigned to fluorine in all of its compounds. For other halogens, $- 1$ is the common oxidation number but it can be positive if they are forming compounds with oxygen.
From the charge neutrality principle, the sum of the oxidation number for all the elements in a given compound would be equal to charge carried by the compound (zero in case it is neutral).

As per the formula s$K{H_2}P{O_3}$, the compound is neutral and contains potassium, hydrogen, phosphorus and oxygen. Let’s assign an oxidation number of x to phosphorus. We can calculate the value of x by using the given formula and the above stated rules as follows:

K{H_2}P{O_3} \to {K^ + } + 2{H^ + } + {P^x} + 3{O^{2 - }}\\\ \left\\{ {1 \times \left( { + 1} \right)} \right\\} + \left\\{ {2 \times \left( { + 1} \right)} \right\\} + \left\\{ {1 \times \left( x \right)} \right\\} + \left\\{ {3 \times \left( { - 2} \right)} \right\\} = 0\\\ 1 + 2 + x - 6 = 0\\\ x = + 3 \end{array}$$ Hence, the oxidation number of $P$ in $K{H_2}P{O_3}$ is determined to be $$ + 3$$ **Hence, the correct option is D.** **Note:** We must keep in mind the balancing of overall charge by following the charge neutrality principle. For neutral compounds the overall charge is equated to zero. For ions, the sum of oxidation states is taken equal to the total charge present on the ion.