Barnes et al$.[26]$ report that in the gas phase the data for combustion of benzene at $1200\backslash$deg Fare best represented by n $=2($a second$-$order reaction$)$ with k $=18$ x $10^6$ cm$*/($mol$.$s$).$

If this relation is correct, and if we start with a gas that is $0\mathrm{.}5$ mol percent $(5000$ ppm$)$

benzene in air, how long will it it take to burn to $0\mathrm{.}0005$ percent $(5$ ppm$)$ benzene?