问题 选择题

已知气体的密度与它的摩尔质量成正比,一只气球若放在空气中静止不动,那么在相同条件下,该气球放在下列气体中会下沉的是(  )

A.O2

B.Cl2

C.H2

D.CO2

答案

一只气球若放在空气(空气平均摩尔质量为29g/mol)中可静止不动,在相同条件下该气球放在密度比空气小的气体中就会下沉,则

A.氧气的摩尔质量为32g/mol,大于空气,则密度比空气大,该气球放在氧气中上升,故A错误;

B.氯气的摩尔质量为71g/mol,大于空气,则密度比空气大,该气球放在氧气中上升,故B错误;

C.氢气的摩尔质量是2g/mol,小于空气,则密度比空气小,该气球放在氧气中下沉,故C正确;

D.二氧化碳的摩尔质量为44g/mol,大于空气,则密度比空气大,该气球放在氧气中上升,故D错误;

故选C.

选择题
阅读理解

Taiwan police cannot decide whether to treat it as an extremely act of stealing or an even cheat. Either way, it could be the perfect crime, because the criminals are birds—homing pigeons !

The crime begins with a telephone message to the owner of a stolen car: if you want the car back, pay up. Then, the car owner is directed to a park, told where to find a bird cage and how to attach money to the neck of the pigeon inside. Carrying the money in a tiny bag, the pigeon flies off.

There have been at least four such pigeon pick-ups in Changwa. What at first seemed like the work of a clever stay-at-home car thief, however,  may in fact be the work of an even lazier and more inventive criminal mind—one that avoids not only collecting money but also going out to steal the car in the first place. Police officer Chen says that the criminal probably has pulled a double trick: he gets money for things he cannot possibly return. Instead of stealing cars, he lets someone else do it and then waits for the car-owner to place an advertisement in the newspaper asking for help.

The thought is supported by the fact that, so far, none of the stolen cars have been returned. Also, the amount of money demanded-under 3,000 Taiwanese dollars–seems too little for a car worth many times more.

Demands for pigeon-delivered money stopped as soon as the press reported the story. And even if they start again, Chen holds little hope of catching the criminal. “We have more important things to do,” he said.

小题1:After the car owner received a phone call, he _______.

A.went to a certain pigeon and put some money in the bag it carried

B.gave the money to the thief and had his car back in a park

C.sent some money to the thief by mail

D.told the press about it小题2:The “lazier and more inventive” criminal refers to _______.

A.the car thief who stays at home

B.one of those who put the ads in the paper

C.one of the policemen in Changwa

D.the owner of the pigeons小题3:The writer mentions the fact that “none of the stolen cars have been returned” to show _______.

A.how easily people get fooled by criminals

B.what Chen thinks might be correct

C.the thief is extremely clever

D.the money paid is too little小题4:The underlined word “they” in the last paragraph refers to ____.

A.criminals

B.pigeons

C.the stolen cars

D.demands for money小题5: We may infer from the text that the criminal knows how to reach the car owners because _______.

A.he reads the ads in the newspaper

B.he lives in the same neighborhood

C.he has seen the car owners in the park

D.he has trained the pigeons to follow them