$[16$ a$]$ A vessel needs to be purged from an initial concentration of $20$ volume percent to $0\mathrm{.}15$ volume

percent oxygen. A vacuum system capable of $9mmHg$ abs$.$ is available. Nitrogen with a $100\%$ purity is

available. Determine the number of purges required assuming atmospheric pressure is $1$atm.

$[16$ b$]$ If the inert in the above operation contains $1200$ ppm oxygen how many purging cycles does it

take to achieve the same oxygen concentration?