What is the oxidation state of phosphorus in ${\text{H}}_3{\text{PO}}_4$?

Solution

1. Let the oxidation state of phosphorus be $x$.
2. The sum of oxidation states in ${\text{H}}_3{\text{PO}}_4$ is $3(+1) + x + 4(-2) = 0$.
3. Solving the equation gives $3 + x - 8 = 0$, so $x=+5$.
4. Hence the answer is (C).