Sodium Sulfate is an ionic compound formed by two ions, Sodium ##Na^+## and Sulfate ##SO_4^-2##.
In order for these two to bond the charges must be equal and opposite. Therefore, it will take two +1 sodium ions to balance the one -2 sulfate ion. This will make the formula for Sodium Sulfate ##Na_2SO_4##.
I hope this was helpful. SMARTERTEACHER
