Question

Diphosphorus pentoxide reacts with water to produce phosphoric acid (`H_3PO_4`). How do you write the balanced equation for this reaction?

Answer 1

`"P"_4"O"_text(10(s]) + 6"H"_2"O"_text((l]) -> 4"H"_3"PO"_text(4(aq])`

Explanation:

The interesting thing about diphosphorus pentoxide, `"P"_2"O"_5`, is that it usually exists as a dimer.

This implies that `"P"_2"O"_5` is actually the compound's empirical formula and that you should use `"P"_4"O"_10` as its molecular formula.

Now, diphosphorus pentoxide reacts violently with water to form phosphoric acid, `"H"_3"PO"_4`. The reaction is highly exothermic and leads to the formation of toxic fumes.

The unbalanced chemical equation for this reaction looks like this

`"P"_4"O"_text(10(s]) + "H"_2"O"_text((l]) -> "H"_3"PO"_text(4(aq])`

To balance this equation, start by multiplying the phosphoric acid by `4` to get equal numbers of atoms of phosphorus on both sides of the equation

`"P"_4"O"_text(10(s]) + "H"_2"O"_text((l]) -> 4"H"_3"PO"_text(4(aq])`

Notice that you have `2` atoms of hydrogen on the reactants' side and `12` on the products' side. Multiply the water molecule by `6` to balance the hydrogen atoms out.

Incidentally, this will also balance out the atoms of oxygen, since you'd now have `16` on the reactants' side and `16` on the products' side.

The balanced chemical equation for this reaction will thus be

`"P"_4"O"_text(10(s]) + 6"H"_2"O"_text((l]) -> 4"H"_3"PO"_text(4(aq])`

It's worth mentioning that diphosphorus pentoxide is a very powerful dehydrating agent.

SIDE NOTE You'll sometimes see this reaction written using the empirical formula of diphosphorus pentoxide, `"P"_2"O"_5`. In that case, the balanced chemical equation will be

`"P"_2"O"_text(5(s]) + 3"H"_2"O"_text((l]) -> 2"H"_3"PO"_text(4(aq])`