To balance a chemical equation, we have to take into account the coefficients and subscripts that indicate us the number of atoms of each element.
The first element we are going to balance is sodium, in the reactants, there is only one atom and in the products there two atoms of Na, so we have to assign 4 as the coefficient of NaOH and 2 as the coefficient of Na2HPO3.
There are 4 atoms of phosphorus in the reactants and 3 of them in the products. To balance it assign 2 as the coefficient of PH3.
Finally, balance the number of oxygens and hydrogens in the equation. There are 5 atoms of oxygen in the reactans and 6 of them in the products. Assign 2 as the coefficient of H2O, at this points, all the elements are balanced.
The balanced equation is:
[tex]P_4+4NaOH+2H_2O\to2PH_3+2Na_2HPO_3[/tex]
