$9\mathrm{.}40.$ Typically $10$ to $30ppm$ of $S{O}_3$ are added to the gas going to an ESP to condition the gas, thereby lowering the resistivity of the collected solid and improving the collection efficiency

$[25].$ If the gas stream is produced by burning a coal with properties as shown in Example $7\mathrm{.}10,$ what fraction of the $S{O}_2$ in the gas would have to be converted to $S{O}_3$ to produce this concentration of $S{O}_3?$