Which oxide produces the most acidic solution when 0.1 mol is added to 1 L of ${\mathrm{H}}_2\mathrm{O}$?

Oxidation state of ${\mathrm{SO}}_3$ is most which is +6. As acidic nature of oxides is proportional to the Oxidation state. So ${\mathrm{SO}}_3$ oxide produces the most acidic solution when 0.1 mol is added to 1 L of 
${\mathrm{H}}_2\mathrm{O}.$