Question

“Na2SO3” represents sodium sulfite.What does the 4 mean in the formula 4Na2SO3?

Answer 1

The mass of substance which consists the equal number of particles as 12 grams of Carbon-12 is said to be mole.

For example:

The balanced chemical equation for production of water from hydrogen and oxygen is:

$2H_2(g) + O_2(g) \rightarrow 2H_2O(l)$

That means 2 moles of $H_2$ reacts with 1 mole of $O_2$ to give 2 moles of  $H_2O$.

The given formula is $4Na_2SO_3$.

Hence, 4 represents the number of moles in $4Na_2SO_3$ formula that is 4 moles of $Na_2SO_3$.

The number of atoms of sodium, $Na$ in $4Na_2SO_3$ is:

$4 \times 2 = 8$

The number of atoms of sulfur, $S$ in $4Na_2SO_3$ is:

$4 \times 1 = 4$

The number of atoms of oxygen in, $O$ in $4Na_2SO_3$ is:

$4 \times 3 = 12$

Answer 2

You would find four put in front of the compound in an equation.  This is to indicate that there are four times more moles in the reaction than there is in the specie that has no number in front of it in the reaction.