(1)NaCl的摩尔质量是 ; 0.1molCO2的质量是 ,它在标准状况下的体积约为 ;在0.5L 0.2 mol·L-1的 Ba(NO3)2溶液中NO3-的物质的量浓度为 。 (2)等质量的SO2和SO3分子数之比为________,同温同压下的密度之比为_________,含氧原子数之比为___________,体积之比为___________.7、By the time I arrived at the hall, the meeting for
half an hour.
A.began
B.had began
C.had been
on
D.has been on