问题 计算题

化学兴趣小组对某工业废水(含有H2SO4、Na2SO4)中H2SO4的含量进行测定。甲、乙两同学各提出不同的测定方法:

(1)甲同学:酸碱中和法

取60g废水于烧杯中,逐渐滴入溶质质量分数为10%的NaOH溶液(反应的化学方程式为:H2SO4+2NaOH==Na2SO4+ 2H2O),反应过程溶液pH变化如右图,求60g废水中H2SO4的质量。

(2)乙同学:沉淀分析法

改用BaCl2溶液代替NaOH溶液测定废水中的H2SO4的含量(反应的化学方程式为:H2SO4+ BaCl2==BaSO4↓+2HCl),你认为结果会       (填“偏高”、“偏低”、“不变”)理由是                                                          

答案

(1) 2.45g

(2)偏高(1分) BaCl2溶液也会与废水中的Na2SO4反应。(2分)

(1)当PH=7时,溶液呈中性,说明两者恰好完全反应,此时加入的NaOH溶液的质量为20克。

解:设废水中硫酸的质量为x

H2SO4+2NaOH=Na2SO4+2H2O

98           80          

x         20g×10%                   

98/80=X/20g×10%   (2分)

X=2.45g            (1分)

答:废水中硫酸的质量为2.45g

(2)偏高,因为 BaCl2溶液也会与废水中的Na2SO4反应。

阅读理解

阅读理解。

     The old man walked with a cane (拐杖) slowly into the restaurant. His poor jacket, patched (打补

丁的) trousers, and worn-out shoes made him stand out from the usual Saturday morning breakfast

crowd. Unforgettable were his pale blue eyes that sparkled like diamonds, large rosy cheeks, and thin

lips that held a steady smile.

      He walked toward a table by the window. A young waitress watched him and ran over to him,

saying, "Here, Sir. Let me give you a hand with that chair."

     Without a word, he smiled and nodded a thank you. She pulled the chair away from the table.

Supporting him with one arm, she helped him move in front of the chair, and get comfortably seated.

Then she pushed the table up close to him, and leaned his cane against the table where he could reach it.

     In a soft, clear voice he said, "Thank you, Miss."

     "You're welcome, Sir." She replied. "My name is Mary. I'll be back in a moment. If you need

anything, just wave at me."

     After he had finished a hearty meal of pancakes, bacon, and hot lemon tea, Mary brought him the

change, helping him up from his chair and out from behind the table. She handed him his cane, and

walked with him to the front door. Holding the door open for him, she said, "Come back and see us,

Sir!"

     He nodded a thank you and said softly with a smile, "You are very kind!"

     When Mary went to clean his table, she was shocked. Under his plate she found a business card and

a note written on the napkin, under which was a $ 100 bill.

     The note on the napkin read, "Dear Mary, I respect you very much, and you respect yourself, too. It

shows by the way you treat others. You have found the secret of happiness. Your kind gestures will shine

through those who meet you."

     The man she had served was the owner of the restaurant. This was the first time that she, or any of his

employees, had seen him in person.

1. Mary ran over to the old man because _____.  

A. the old man was the boss of the restaurant 

B. she was worried that the old man might cause trouble to the restaurant 

C. she saw the old man had some difficulty moving and taking a seat

D. the old man had asked her to wait on him

2. The man came to the restaurant ______. 

A. to have breakfast  

B. to see his employees  

C. to find out how his restaurant was working 

D. to see how Mary served customers

3. The words the man left on the napkin suggested that _____. 

A. respecting others means respecting oneself  

B. serving others is a respectable job 

C. Mary would get a rise as a result of her kindness 

D. Mary's kind service would bring in more money for her

4. Which of the following titles goes best with the story? 

A. A kind- hearted Girl 

B. Shining Kindness 

C. Kindness Means Opportunities 

D. A Special Customer

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