问题 选择题

小东欲测定某氯化钡样品中氯元素的质量分数,取该样品(杂质不溶于水也不与酸反应且不含氯元素)50g,加入足量的稀硫酸充分反应后过滤,得干燥滤渣55g.则该样品中氯元素的质量分数为(  )

A.7.1%

B.28.4%

C.35.5%

D.42.6%

答案

设氯化钡的质量为是x

BaCl2+H2SO4═BaSO4↓+2HCl固体变化的质量

208 23325

x 55g-50g=5g

208
x
25
5g

解得:x=41.6g

则氯元素的质量分数:

41.6g×
35.5×2
208
50g
×100%=28.4%

分析所给的选项可以知道选项B是正确的.

故选B.

单项选择题

How to protect children Web fans from unsuitable material on-line while encouraging them to use the Internet has long been discussed in the US.

For some parents, the Internet can seem like a jungle, filled with danger for their children. But jungles contain wonders as well as dangers and with good guides, some education, and a few precautions, the wilds of the Internet can be safely navigated. "Kids have to be on-line. If we tell our kids they can’t have access to the Internet, we’re cutting them off from their future, " said an expert.

Most kids have started to use search engines. Many of them are great for finding tons of interesting Internet sites, and they can also locate places where you might not want your kids to go. There are search engines designed just for kids. A certain software contains only sites that have been selected as safe. The most popular way to limit access would be to use what is known as a "content screener". But this can’t be wholly reliable, and the best thing parents can do is to talk to their kids and let them know what is OK or not OK to see or do on the Internet. Another way is that mum or dad is nearby when the child is surfing the Internet.

A few other tips

·Don’t put the PC in a child’s room but keep it in an area where mum or dad can keep an eye on things. That also makes the Internet more of a family activity.

·Ask your child what he or she has been doing and about any friends they make on-line.

·Tell your child not to give on-line strangers personal information, especially like address and phone number.

·And tell your children never to talk to anyone they meet on-line over the phone, send them anything, accept anything from them or agree to meet with them unless you go along.

According to the passage, we can infer that()

A. soft wares fit for children want programming

B. a child who is on-line is in danger

C. Internet is a jungle full of danger

D. Internet contains a lot of harmful sites

单项选择题