问题 计算题

(3分)某盐矿井采出的食盐(俗称井盐)中含有CaCl2,为设计除杂方案需测定CaCl2的含量,准确称量上述井盐22.8g,向其中加入100g质量分数为10.6%的Na2CO3溶液恰好完全反应,且所得食盐溶液仍为不饱和溶液。求:

(1)井盐中CaCl2的质量分数。

(2)检测后的溶液中NaCl的质量分数。

答案

(1)48.7% (2)20.7%

题目分析:(1)要求井盐中CaCl2的质量分数,可根据化学方程式以及Na2CO3的质量,计算出井盐中CaCl2的质量,然后再根据质量分数公式就可求出井盐中CaCl2的质量分数;

解:设22.8g井盐中含CaCl2的质量为X,加入Na2CO3溶液后生成NaCl的质量为Y,CaCO3沉淀的质量为Z。

CaCl2 +  Na2CO3 =  CaCO3↓ +  2NaCl

111      106          100      117

X       100g×10.6%    Z       Y

111:106= X:100g×10.6%

106:100=100g×10.6%:Z

106:117=100g×10.6%:Y

解得X=11.1g,Y=11.7g,Z=10g 

所以CaCl2的质量分数=="48.7%"

(2)欲知检测后溶液中NaCl的质量分数,须知检测后NaCl的质量及溶液质量,NaCl的质量要包括原来的氯化钠和反应生成的氯化钠,溶液质量为总质量-生成沉淀的质量,再根据溶质质量分数公式计算即可。

由上面的计算可知,22.8g井盐中含NaCl的质量为22.8g-11.1g=11.7g;

故反应后溶液中含NaCl的质量为11.7g+11.7g=23.4g

所以溶液中NaCl的质量分数为="20.7%"

答:井盐中CaCl2的质量分数为48.7%,检测后的溶液中NaCl的质量分数为20.7%

点评:此题是关于化学方程式及溶质质量分数的计算题,主要是利用反应方程式来分析和解决化学计算中的有关问题,要求学生要有较强的分析能力,解题的关键是要明确所求溶液的溶质是什么,有几部分组成;然后根据相关的化学反应方程式,并依据已知量对未知的应求量进行求解计算即可,注意解题要规范。

选择题
阅读理解

For better or worse, multiple marriages aren't just for actress Elizabeth Taylor (famous for her eight marriages) anymore.

More Americans than ever are tying the knot for the third time or more.

Lynn Y. Naugle, a 53-yeap-old family therapist in New Orleans, says that people's personal needs and desires simply change as their life evolves.

"What functions well in the first part of our lives may not function well in the second or third part of our lives," she explains.The first marriage lasted 21 years, her second marriage five years.Two years ago, she wed for a third time, and she describes this union as an "extremely easy marriage".

Today, at an estimated one of seven weddings, the bride, the groom or both are making that trip down the aisle for at least the third time.That's twice as many as a generation ago, according to the US National Centre for Health Statistics.

In part, the sudden change in multiple marriages is a side effect of the 1970s divorce increase that has supplied an ever expanding pool of divorced singles.Even the simple fact that people are living longer has opened the door to marrying more often.No fault divorce laws (meaning no one is blamed for the failure of the marriage), and cultural changes have also meant there's less pressure than in past generations to stay in a joyless or abusive marriage.

While a single divorce didn't block either Ronald Reagan or Bob Dole from seeking the most highly demanding job in America—the US presidency—modern society still raises an; eyebrow at more than one

marriage mistake.

Indeed, there are signs that attitudes are changing."It's coming out of the closet or becoming more accepted," says Glenda Riley, who wrote a book on the history of divorce in the US."There's still embarrassment on the personal level, while there is growing acceptance on the public level for three or more marriages in a lifetime."

There is no guarantee, of course, that the third time is the best.To the contrary, second and third marriages run an equal or greater risk of divorce than first marriages, which today are given 4 out of 10 odds of failing, and they tend to end more quickly.Divorce statistics show that failed second marriages typically end two years sooner than first marriages, lasting six years on average rather than eight.That leaves some doubly divorced people open for a third try at a relatively young age.

小题1:What does the underlined phrase "tying the knot" (Para.2) mean?

A.Getting married.

B.Getting engaged.

C.Having babies.

D.Attending funerals,小题2:What is NOT the reason for the increase in multiple marriages according to the passage?

A.People are healthier and enjoy a longer life than ever before.

B.Many people have become single after a divorce boom in the past years.

C.There is no divorce law restricting people to getting divorced.

D.People have less pressure to leave a joyless marriage.小题3:What can we learn from the seventh paragraph?

A.Ronald Reagan and Bob Dole failed in seeking the job (the US presidency) because they were divorced.

B.Ronald Reagan and Bob Dole succeeded in seeking the job (the US presidency) because they were not divorced.

C.Modern society accepts multiple marriages completely.

D.There is still prejudice against multiple marriages in modern society.小题4:Which of the statement is WRONG?

A.Acceptance to multiple marriages is different on personal level and public level.

B.Because second marriages end sooner than first ones, people get married for the third time at a relatively young age.

C.People learn from experience so that a second or third marriage is more stable

D.The first marriage lasts eight year on average.