$2.$ Use a vacuum purging technique to purge oxygen from a $150-$ft$3$ tank containing air. Reduce the oxygen concentration to $1\%$ using pure nitrogen as the inert gas. The temperature is $80\backslash$deg F$.$ Assume that the vacuum purge goes from atmospheric pressure to $20$ mm Hg absolute$.$ Determine the number of purge cycles required and the total moles of nitrogen used.