Question

What is the chemical equation for diphosphorus trioxide + water ---> phosphorous acid?

Answer 1

Your answer is $P_{2}O_{3}+3H_{2}O\rightarrow\ 2H_{3}PO_{3}$

but if you are looking for some explanation check description below :)

Diphosphorus trioxide formula is:

$P_{2}O_{3}$

Di stands from 2 atoms of something, in this case we have 2 atoms of Phosphorus.

Trioxide means that we have 3 atoms of oxygen in this substance that's why we have: $P_{2}O_{3}$

And water is just $H_{2}O$

Unbalanced formula is:

$P_{2}O_{3}+H_{2}O\rightarrow\ H_{3}PO_{3}$

Now we need to balance this equation

On left hand side we have 2 atoms of Phosphorus, on right hand side we have just one atom of Phosphorus, that's why we need to put 2 before $H_{3}PO_{3}$

Now we need to count all atoms of oxygen and hydrogen and make it equal on both side.

So we need to put 3 on left hand side before water to make this equation balanced.
That's why correct answer is:
$P_{2}O_{3}+3H_{2}O\rightarrow\2H_{3}PO_{3}$