The charge on the tin ion is the same as the Roman numeral in the Stock name.
SnO₃ = tin(VI) oxide
Sn₂O₅ = tin(V) oxide
SnO₂ = tin(IV) oxide
Sn₂O₃ = tin(III) oxide
SnO = tin(II) oxide
Sn₂O = tin(I) oxide
Each oxide ion has a charge of -2.
To get the charge on a tin ion, you calculate the total charge on the O ions. Then you change the sign and divide by the number of Sn ions.
E.g., for Sn₂O₅: Charge on 1 O = -2; charge on 5 O = -10; charge on 2 Sn = +10; charge on 1 Sn = +5.
