The following gas-phase reaction is operated isothermally and isobarically in a BR reactor mounted with a piston at a temperature of 350C and a pressure of 1.0 atm: 2A-> B (-rA)=KCa^0.5, k=0.1 (mol^0.5)/(L^0.5).s . Initially, the reactor has been charged with pure A. Estimate the time needed to achieve a conversion of 0.7.