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Write the chemical equation for the preparation of phosphoric acid (${H_3}P{O_4}$) from orthophosphorus acid (${H_3}P{O_3}$) ?

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Hint: The molecular formula of phosphoric acid shows that it has one atom greater in number of oxygen than orthophosphorus acid. Thus, by addition of oxygen atom, one can get phosphoric acid.
Complete step by step answer:
The orthophosphorus acid is the common name. In IUPAC nomenclature, it is called Phosphonic acid. The ${H_3}P{O_3}$ is a diprotic acid i.e. it can donate only two protons. Thus structurally, the formula for orthophosphorus acid can be written as $HPO{(OH)_2}$.
This is not so stable in nature and reacts easily with the moisture in the air.
The phosphoric acid is a weak triprotic acid. There are various methods for its synthesis. It can be even prepared from orthophosphorus acid as -
${H_3}P{O_3} + \frac{1}{2}{O_2} \rightleftharpoons \;{H_3}P{O_4}$
The orthophosphorus acid gets converted to phosphoric acid in the presence of oxygen which is present in moisture. This reaction is reversible in nature. There is another alternative which is to heat orthophosphorus acid above ${180^0}$ C temperature. Even on heating at such a high temperature also, the orthophosphorus acid gets converted to phosphoric acid.
Note:
The orthophosphorus acid is unstable because in the presence of moisture, it gets converted to phosphoric acid. The phosphoric acid is stable in nature. It is a viscous liquid which has no colour and even no odour. Even the phosphoric acid is a weak acid in nature but it is stable. Its solution which is 85 % concentrated can produce irritation on eyes and skin.