问题 计算题

某化学活动小组欲用纯碱制取烧碱,做了如下实验:

现取20g纯碱样品(假设杂质不溶于水,也不与其他物质反应),向其中加入34.1g水使其完全溶解,然后再加入100g的溶质质量分数为l7.1%的氢氧化钡溶液,至恰好完全反应,过滤得到滤渣和一种不饱和溶液。请回答问题:

(1)上述实验步骤中,请指出操作中的错误                                  

(2)请写出发生反应的化学方程式                                         

(3)列出参加反应固体物质质量(x)的比例式为                    

(4)求纯碱样品的纯度为                 

(5)反应后所得溶液的溶质质量分数为             

答案

(1)过滤时没用玻璃棒引流 (2)Na2CO3+ Ba(OH)2==BaCO3↓+2NaOH

(3) (4)53%    (5)6.4%

题目分析:(1)过滤时玻璃棒的作用是引流;(3)根据化学方程式的计算,比例式为;(4)根据(2)问求出参加反应的碳酸钠的质量,为20g纯碱样品中纯净的碳酸钠的质量;(5)根据溶液质量分数的定义,得用溶质质量除以溶液总质量然后乘以百分之百,溶质质量即为反应生成的氢氧化钠的质量,根据(2)中化学方程式可以算出,溶液质量即为20g纯碱样品中纯净的碳酸钠的质量加上34.1g水,再加上100g的溶质质量分数为l7.1%的氢氧化钡溶液的质量,最后减去生成的碳酸钡的质量(根据(2)中化学方程式可以算出)。

点评:这类题目要有层次的做,不要浮躁,属于中考必考题型,注意计算细心。

阅读理解

阅读理解。

     The word "day" has two meanings. When we talk about the number of days in a year, we are using "day"

to mean 24 hours. But when we talk about day and night, we are using "day" to mean the time between sunrise

and sunset. Since the earth looks like a ball, the sun can shine on only half of it at a time. Always one half of

the earth is having day and the other half night. A place is moved from day into night and from night into day

over and over by the spinning (旋转) of the earth. At the equator (赤道) day and night are sometimes the same

length (长度). They are each twelve hours long. The sun rises at 6 o'clock in the morning and sets at 6 o'clock

in the evening. For six months the North Pole is tilted (倾斜) toward the sun. In those months the Northern

Hemisphere (半球) gets more hours of sunlight than the Southern Hemisphere. Days are longer than nights.

South of the equator nights are longer than days. For the other six months the North Pole is tilted away from

the sun. Then the Southern Hemisphere gets more sunlight. Days are longer than nights. North of the equator

nights are longer than days. Winter is the season of long nights. Summer is the season of long days.

1. When the Western Hemisphere is having day, the Eastern Hemisphere is having ______. [ ]

A) both day and night

B) day

C) night

2. A place is moved from day into night and from night into day over and over by ______ of the earth. [ ]

A) the pushing

B) the pulling

C) the spinning

3. At the equator day is as long as night ______.[ ]

A) sometimes

B) never

C) usually

4. When the North Pole is tilted toward the sun, the Northern Hemisphere gets ______ sunlight. [ ]

A) less

B) more

C) all

5. When it is winter in China, ______. [ ]

A) the USA is tilted toward the sun  

B) the South Pole is tilted away from the sun  

C) the North Pole is tilted away from the sun

单项选择题