问题 问答题

25℃时,将a g碳酸钠(化学式为Na2CO3)溶解于100g水中恰好形成饱和溶液.所得溶液的溶质质量分数为______,此温度下碳酸钠的溶解度为______g.

向此溶液中滴入足量稀盐酸,反应的化学方程式为______.

答案

由题意得,溶液中溶质质量为ag,溶剂质量为100g,溶液质量为100g+ag,溶质质量分数=

溶质质量
溶液质量
×100%=
ag
100g+ag
×100%=
a
100+a
×100%;

由溶解度的定义可知,100g水中最多溶解的溶质质量即是此物质该温时的溶解度,题目水恰为100g溶解的溶质质量为ag,且恰饱和,故此时其溶解度为ag;

碳酸钠与酸发生复分解反应,相互交换成分得氯化钠与碳酸,碳酸不稳定分解为水与二氧化碳,反应条件为常温,依据质量守恒定律配平,最后标注气体符号,即得反应的化学方程式:Na2CO3+2HCl=2NaCl+H2O+CO2↑.

故答案为:

a
100+a
×100%;     a;   Na2CO3+2HCl=2NaCl+H2O+CO2↑.

阅读理解

Every year thousands of people get hurt or die when they are crossing the road. Most of these people are old people and children. Old people often get hurt or die because they can’t see or hear very well. Children usually meet with accidents because of their carelessness. They forgot to look and listen before they cross the road.

How can we lessen (减少) traffic accidents? All of us must obey the traffic rules. For the drivers, they shouldn’t drive too fast. If they drive too fast, it will be very difficult to stop the cars in a very short time. For the pedestrians (行人), it’s very important to be careful when they are walking on the road. Therefore, when we walk across the road, we must try to walk along the pavements(人行道). We must stop and look both ways before crossing the road. Look left first, next look right, then look left again. Only when we are sure that the road is clear, we can cross it. The right way to cross the road is to walk quickly. It’s not safe to run. If people run across the road, they may fall down. Teens(青少年) should try to help children, old people or blind people to cross the road, and never play in the street.

小题1:Thousands of people get hurt or die when they are crossing the road every year, most of them are ______.

A.children and drivers

B.old people and children

C.old people and blind people

D.teens and drivers小题2:To lessen traffic accidents, we shouldn’t ______.

A.run quickly

B.walk quickly

C.be sure that the road is clear

D.look left, look right and look left again小题3: According to the passage, we know that ______ will be safe.

A.driving too fast

B.running across the road

C.walking alone the pavements

D.playing in the street小题4: Why do old people often get hurt or die when they are crossing the road?

A.Because they don’t look and listen.

B.Because they walk too fast.

C.Because they are not careful.

D.Because they can’t see or hear very well.小题5: What is the main idea of this passage?

A.Never play in the street.

B.Being safe in the street.

C.Look and listen before we cross the road

D..Stop and look both ways before crossing the road.

多选题