问题 问答题

向40克已知过量的氢氧化钡溶液中,加入25%的硫酸钠溶液28.4克,反应完全后,过滤沉淀;再滴加30毫升20%的稀硫酸(密度为1.147克/厘米3)恰好使滤液呈中性.求原氢氧化钡溶液的溶质质量分数.

答案

设25%的硫酸钠溶液28.4克完全反应消耗氢氧化钡的质量为x,生成NaOH质量为y

28.4g25%的硫酸钠溶液中Na2SO4的质量=28.4g×25%=7.1g

Na2SO4+Ba(OH)2=BaSO4↓+2NaOH

142               171              80

7.1g               x               y

142
7.1g
=
171
x
 解之得x=8.55g

142
7.1g
=
80
y
 解之得y=4g

设4gNaOH消耗硫酸的质量为z

2NaOH+H2SO4=Na2SO4+2H2O

80                98

4g                z

80
4g
=
98
z
 解之得z=4.9g

则所滴加的硫酸中与氢氧化钡反应的质量=30mL×1.147g/cm3×20%-4.9g=1.982g

设与硫酸恰好完全反应的氢氧化钡质量为w

H2SO4+Ba(OH)2=BaSO4↓+2H2O

98                   171

1.982g               w

98
1.982g
=
171
w
 解之得w≈3.46g

原氢氧化钡溶液的溶质质量分数=

8.55g+3.46g
40g
×100%≈30%

答:原氢氧化钡溶液的溶质质量分数约为30%.

阅读理解

阅读理解。

     Luo Gaoqi hopes to join the increasing number of Chinese students in the United States because he wants

the experience of studying in a foreign land as it will help his future job at home."I want to try Western

education because people there think differently," said the 21-year-old who is in his final year of undergraduate

studies. "I also hope to meet friends from different cultures. The social connections may help in the future."

     The latest report from the US-based institute of International Education said China, for the first time, has

become the top country of origin for international students in the United States. For Yang Fan, a journalism

(新闻专业) student, said studying in the United States is a way to escape the competition for places in China's

top universities.

     "Due to limited education resources, only about 60 percent of high school graduates in China enter college,

and a much lower rate enters the top-level ones. "A foreign degree will definitely help when I seek a job in

China," Yang said.

     Both Yang's and Luo's parents are supportive of them studying abroad. Neither of them is thinking of

settling down in the United States after graduation.

     "It is not realistic to do so. As a journalism student, there would be no advantage for me to work in the

United States," Yang said. "But in China, with a US degree and good English, I may stand out against other

students who have only studied in China."

     China still needs to improve its environment for scientists and high-level talents, said Xiao Mingzheng,

director of the Center for Human Resource Development and Management Research of Peking University,

adding that, in particular, the country needs to improve its policies relating to research environments.

      In June, the Chinese government published a plan concerning talent development for the next 10 years. In

it, it has improved policies and increased budgets to attract talented students back home.

1. Why does Luo Gaoqi wants to study in a foreign land according to his words?

A. Because the people there are more intelligent.

B. Because the foreign countries are more developed.

C. Because he can make more friends there.

D. Because the culture there is different from that in China.

2. What do Guo Gaoqi and Yang Fan have in common?

A. Their major is the same, which is journalism.

B. Both of them want to go back to China after graduation.

C. Their parents are against their decision.

D. They are forced to study in a foreign land by their parents.

3. From the passage, we can infer that _______.

A. scientists and high-level talents need a better environment in China

B. the students abroad are unwilling to go back to China

C. China doesn't need those people to come back to our country at all

D. we all need to go abroad to study and then serve our own country

判断题