问题 问答题

配制质量分数为3% 的氯化钠溶液:

(1)用已配好的质量分数为6%的氯化钠溶液,配制50g质量分数为3% 的氯化钠溶液.由浓溶液配制稀溶液时,计算的理论依据是______.

(2)计算:

所需6%的氯化钠溶液______ g;水______g.量水时,量筒必须平放,视线要跟量筒凹液面的______处保持水平.若仰视,会导致配得的溶液质量分数______(填“偏大”或“偏小”).

答案

(1)配制50g质量分数为3% 的氯化钠溶液,所需要的溶质源于6%的氯化钠溶液;

(2)设需要6%的氯化钠溶液的质量为x,则有50g×3%=x×6%,解得x=25g,需要6%的氯化钠溶液25g,则还需要水的质量是50g-25g=25g;  在量取一定体积液体时,视线要跟凹液面的最低处保持水平,若仰视读数,则会导致实际水的体积比所需体积大,溶剂质量偏大,溶液质量偏大,溶质的质量分数偏小;

故答案为:

(1)浓溶液和稀溶液溶质的质量相等;

(2)25;25;最低处;偏小.

单项选择题 案例分析题

Questions 11 to 15 are based on the following passage.

Scratchy throats, stuffy noses and body aches all spell misery, but being able to tell if the cause is a cold or flu (流感) may make a difference in how long the misery lasts.

The American Lung Association (ALA) has issued new guidelines on combating colds and the flu, and one of the keys is being able to quickly tell the two apart. That’s because the prescription drugs available for the flu need to be taken soon after the illness sets in. As for colds, the sooner a person starts taking over-the-counter remedy, the sooner relief will come.

The common cold and the flu are both caused by viruses. More than 200 viruses can cause cold symptoms, while the flu is caused by three viruses—flu A, B and C. There is no cure for either illness, but the flu can be prevented by the flu vaccine (疫苗), which is, for most people, the best way to fight the flu, according to the ALA.

But if the flu does strike, quick action can help. Although the flu and common cold have many similarities, there are some obvious signs to look for.

Cold symptoms such as stuffy nose, runny nose and scratchy throat typically develop gradually, and adults and teens often do not get a fever. On the other hand, fever is one of the characteristic features of the flu for all ages. And in general, flu symptoms including fever and chills, sore throat and body aches come on suddenly and are more severe than cold symptoms.

The ALA notes that it may be particularly difficult to tell when infants and preschool age children have the flu. It advises parents to call the doctor if their small children have flu-like symptoms.

Both cold and flu symptoms can be eased with over-the-counter medications as well. However, children and teens with a cold or flu should not take aspirin for pain relief because of the risk of Reye syndrome (综合症),a rare but serious condition of the liver and central nervous system.

There is, of course, no vaccine for the common cold. But frequent hand washing and avoiding close contact with people who have colds can reduce the likelihood of catching one.

 

Which of the following symptoms will distinguish the flu from a cold? ()

A.A stuffy nose.

B.A high temperature.

C.A sore throat.

D.A dry cough.

单项选择题