根据PV=nRT算出n

v=1500km^3=1.5*10^9m^3
T=15+273=288K
P=789.0 torr＝101325＊789/760＝105191Pa
n=PV/RT=105191*1.5*10^9/(8.31*288)=6.6*10^10 mol