The formula determined was:
Na
2
SO
4
but why couldn't one say the formula was:
Na
4
S
2
O
8
or
Na
6
S
3
O
12
or
Na
8
S
4
O
16
etc.
There is no indication here unless one knew the molar mass. Thus, one say that Na
2
SO
4
is the "simplest" or "empirical" formula.