问题 问答题

已知空气的密度是1.293克/升,氧气的密度是1.429克/升,计算空气中氧气的质量百分含量?(即质量分数)

答案

设空气的体积为1L,因为氧气在空气中的体积分数为21%,所以氧气体积为0.21L,

空气的质量=1L×1.293g/L=1.293g,

氧气的质量=0.21L×1.429g/L=0.3g,

所以空气中氧气质量分数=

0.3g
1.293g
×100%=23.2%.

答:空气中氧气的质量百分含量为23.2%

故答案为:23.2%

单项选择题

The day of terror at the Virginia Polytechnic and State University in Blacksburg began at about 7:15 a. m. , with the shooting of a woman and a male resident adviser on the fourth floor of a dorm building on campus, Kristen Bensley, a freshman who lived below the floor where the shooting occurred, told TIME, "There were minors going on about the assailant was fighting with his girlfriend or something of that nature." Bensley notes that only residents can get into the building, using a specific "passport", that is, a card that one has to swipe in order to open doors before 10 a.m. If he was an outsider, someone would have had to let him in. Or more likely, he was a resident of the dorm himself. If so, how did be keep so much ammunition unnoticed

Unlike high schools, most universities can’t beef up security with a metal detector or two. So what can be done to protect students Other questions remain unanswered. Why was there a two-hour gap between the incident at the dorm and a far more fatal one across campus At one point, that led to theorizing that more than one gunman was involved. The gunman who killed at least 30 people at Norris Hall shortly after 9 a.m. was described by some sources as an Asian man.

It has been a surreal time for the students. Brandon Stiltner, a senior aerospace engineering student, and Jonathan Hess, a senior mechanical engineer, were watching TV all day but by noon they’d had enough. "We decided we needed to do something," Stiltner said. "We were worthless sitting around." So they took their six-foot Virginia Tech sign off the wall and logged into Facebook. Within the next few hours 100 people replied to their e-mail request for a vigil.

By 8 p.m. hundreds bf students began filing down the steps of the War Memorial Chapel toward the drill field. Clusters of two and three students stood together in silence. Slowly they began to line up to sign the board. "I’m still really in disbelief," says Stiltner. The shock of the day’s shootings sank in, Hess said, as he carried the sign across campus for the vigil. "It hit me," Hess said, "to know that it was in these buildings." The media crews that swarmed campus were also surreal to Hess and Stiltner. "We could look out our window and see exactly what’s on TV," Stiltner says. He watched his sign crowded with initials and prayers, awaiting the names of the victims, He shuddered. "I hope I don’t have any nasty surprises.

From this article we can guess that the author is a()

A. reviewer

B.journalist

C.observer

D. novelist

多项选择题