Why is sodium sulfate's formula Na2SO4?

Answer 1

Na has a charge of +1 and SO4 has a charge of -2 so to balance it out, you need 2 Na.

Answer 2

Sodium sulfate's formula is Na2SO4 because it contains two sodium ions (Na+) for every one sulfate ion (SO4 2-). This combination of ions results in a neutral compound where the total positive charge from the sodium ions balances the total negative charge from the sulfate ion.