问题 选择题

为了测定铜铁合金中铜的质量分数,在10.00g试样中加入200mL、0.6mol/L的稀硝酸,充分反应后剩余金属7.48g,再向其中加入50mL、0.4mol/L的稀硫酸,充分振荡后剩余金属6.60g。若硝酸的还原产物只有NO,下列说法正确的是

A.上述测定不能达到实验目的

B.剩余的7.48g金属为铜

C.共生成NO气体 0.04mol

D.该合金中铜的质量分数为69.2%

答案

CD

题目分析:铁的金属性强于铜的,所以硝酸首先氧化金属铁,第一次反应后金属过量,硝酸不足,说明生成物一定有硝酸亚铁。加入稀硫酸后金属又溶解了7.48g-6.60g=0.88g,硫酸中氢离子的物质的量是0.05L×0.4mol/L×2=0.04mol,则根据方程式3M+2NO3+8H=3M2+2NO↑+4H2O可知,NO的物质的量是0.01mol,参加反应的金属的物质的量是(0.04mol÷8)×3=0.015mol,则M的相对原子质量是0.88÷0.015=58.67,这说明溶解的金属是铁和铜,所以剩余的7.84g金属是铁和铜的混合物,因此第一次溶解的金属铁是10.00g-7.48g=2.52g,物质的量是2.52g÷56g/mol=0.045mol,6.60g全部是金属铜。硝酸的物质的量是0.2L×0.6mol/L=0.12mol,根据方程式3Fe+8HNO3=3Fe(NO3)2+2NO↑+4H2O可知,生成物硝酸亚铁的物质的量是0.045mol,NO的物质的量是0.03mol,因此共放出NO是0.04mol。设第二次消耗金属铁和铜的物质的量分别是x和y,则56x+64y=0.88、2x+2y=0.01×3,解得x=0.01mol、y=0.005mol,所以合金中铜的质量是6.60g+0.005mol×64g/mol=6.92g,所以该合金中铜的质量分数为69.2%,答案选CD。

单项选择题
阅读理解

     All across the country, kids are heading back to school. Despite advice from the nation's secretary

of health and human services, recess(课间休息) and physical education have in many schools been

cut back or canceled. While most of kids are eager to catch up with friends, the general feeling is that

"playtime is over."

     Summer vacation does offer most children a bit of free playtime — the sort that leads to physical

activity and self-organized play, often in short supply during the school year. Still, it's not enough.

Playtime shouldn't be limited to summer vacation: it's important all year.

     For most American children in the past, "going out to play" was the norm. Today, according to a

University of Michigan study, children spend 50 percent less time outside than they did just 20 years

ago. There is a direct line between lack of play and some frightening public health and social trends:

tragic statistics for obesity(肥胖), 4.5 million children diagnosed with attention deficit disorder(ADD)

(儿童多动症), an increase in childhood depression and classroom behavioral problems, and an

inability to interact well with peers.

     Just an hour a day of active play-running, chasing, games like dodge ball—can provide intense skill

learning. Physical activity is known to lessen the symptoms of mild attention deficit disorder, and

reduces the risk of childhood obesity. Active kids can perform better academically in the long term.

     We should be aware of how important play is to one's development. Even a play-deprived social

mammal(哺乳动物) like a rat or monkey will become unable to tell friend from enemy, and poor at

handling stress when it enters adulthood. My studies of young murderers revealed that most had lacked

normal, developmentally appropriate play as children and pre-adolescents.

     Play is fun, but ultimately it is also useful. The smarter the animal, the more they play. For humans,

play refreshes us because it gets us in touch with our core selves and the joy of life.

     I feel sad about the kids who have given up on play for another nine months. Their devaluation of

play is a tragic loss for them and for our society.

1. Children lacking playtime may have the following problems EXCEPT__________.  

A. to become fat  

B. to suffer from depression  

C. to fail to get along well with others  

D. to murder their peers

2. Active play is beneficial to children because __________.  

A. it inspires children to catch up with their friends

B. it is the best way to cure children with obesity  

C. it helps children to improve academic achievement

D. it can teach children to tell friend from enemy

3. "A play-deprived social mammal" refers to an animal that__________.  

A. has lost chance to play   

B. requires enough playtime

C. enjoys playing very much  

D. knows the importance of play

4. Which of the following statements is TRUE?  

A. The article was written before the summer vacation.  

B. American children lack playtime during the school year.  

C. Schools give children more playtime in the new semester.  

D. Children with ADD should play less than normal children.

5. Which of the following will be the best title?  

A. Summer Vacation is over  

B. What Playtime Brings to Children  

C. Lack of Play Leads to Problems

D. Let the Children Play