问题 计算题

(5分)将6.5g石灰石样品投入到36.5g稀盐酸溶液中制取二氧化碳气体,恰好完全反应。反应后将溶液过滤、干燥,称量剩余固体残渣的质量为1.5g(提示:①石灰石样品中的杂质不溶于水,也不发生反应;②反应后生成的氯化钙完全溶解)。计算:

(1)该石灰石样品中碳酸钙的质量是多少?

(2)反应后所得溶液的溶质质量分数为多少?(计算结果保留一位小数)

答案

解:(1)碳酸钙的质量:6.5g-1.5g=5g………………………………(1分)

(2)设反应后生成的氯化钙质量为x,生成的二氧化碳质量为y:

CaCO3+2HCl=CaCl2+CO2↑+H2O…………………(1分)

100        111  44

5g         x   y

=    x=5.55g

=     y=2.2g………………………………(1分)

反应后所得溶液质量为:5g+36.5g-2.2g=39.3g

反应后所得溶液的溶质质量分数为:×100%=14.1%………………(1分)

答:该石灰石样品中碳酸钙的质量为5克,反应后所得的溶液的溶质质量分数为14.1%。………………………………………(设问和答共1分)

其它合理解答均可得分。

分析:(1)由样品和剩余固体的质量即可求得该石灰石样品中碳酸钙的质量;

(2)由反应的碳酸钙的质量,根据反应方程式即可求得生成氯化钙和二氧化碳的质量;再根据质量守恒定律即可求得反应后溶液的质量;最后根据溶质质量分数=×100%即可求得反应后所得溶液的溶质质量分数.

解:(1)碳酸钙的质量:6.5g-1.5g=5g;

(2)设反应后生成的氯化钙质量为x,生成的二氧化碳质量为y:

CaCO3+2HCl=CaCl2+CO2↑+H2O

100        111   44

5g          x    y

=    x=5.55g

=     y=2.2g

反应后所得溶液质量为:5g+36.5g-2.2g=39.3g;

反应后所得溶液的溶质质量分数为:×100%=14.1%;

答:该石灰石样品中碳酸钙的质量为5克,反应后所得的溶液的溶质质量分数为14.1%.

点评:求反应后所得溶液的质量的计算是初中化学计算的一个重点内容,其方法一般是:所加入的所有物质的质量总和-生成气体的质量-生成沉淀的质量.

单项选择题
阅读理解

阅读理解。

      Believe it or not, optical illusion (错觉) can cut highway crashes.

     Japan is a case in point. It has reduced automobile crashes on some roads by nearly 75 percent

using a simple optical illusion. But stripes, called chevrons (人字形), painted on the roads make drivers

think that they are driving faster than they really are, and thus drivers slow down. Now the American

Automobile Association Foundation for Traffic Safety in Washington D.C. is planning to repeat

Japan's success. Starting next year, the foundation will paint chevrons and other patterns of stripes

on selected roads around the country to test how well the patterns reduce highway crashes.

     Excessive (too great) speed plays a major role in as much as one fifth of all fatal traffic accidents,

 according to the foundation. To help reduce those accidents, the foundation will conduct its tests in

areas where speed-related hazards (danger) are the greatest curves, exit slopes, traffic circles, and bridges.

Some studies suggest that straight, horizontal bars painted across roads can initially cut the average speed

of drivers in half. However, traffic often returns to full speed within months as drivers become used to

seeing the painted bars.

     Chevrons, scientists say, not only give drivers the impression that they are driving faster than they

really are but also make a lane appear to be narrower. The result is a longer lasting reduction in highway

speed and the number of traffic accidents.

1. The passage mainly discusses _______.

A. a new way of highway speed control

B. a new pattern for painting highways

C. a new way of training drivers

D. a new type of optical illusion

2. On roads painted with chevrons, drivers tend to feel that _______.

A. they should avoid speed-related hazards

B. they are driving in the wrong lane

C. they should slow down their speed

D. they are coming near to the speed limit

3. The advantage of chevrons over straight, horizontal bars is that the former _______.

A. can keep drivers awake

B. can cut road accidents in half

C. will look more attractive

D. will have a longer effect on drivers

4. The American Automobile Association Foundation for Traffic Safety plans to _______.

A. try out the Japanese method in certain areas

B. change the road signs across the country

C. replace straight, horizontal bars with chevrons

D. repeat the Japanese road patterns