How do you balance #Mg + H_3PO_4 -> Mg_3(PO_4)_2 + H_2#?
Balancing chemical equations always involves a bit of algebra, and maybe some trial-and-error. The objective is always to ensure that the numbers (moles) of each element are the same on each side of the equation.
In this case it is a redox reaction, so sometimes the use of the oxidation state changes can help. First, the phosphate ion is not changing, just switching partners. The magnesium is being oxidized (
The key changes are:
So, there must be twice as many
That means the
Put them all together and check the