问题 计算题

(本题共6分)

某学生课外兴趣小组为了测定镁铝合金中铝的含量,进行了如下实验。将合金3.0克投入到足量的100 mL 1.5 mol•L-1的烧碱溶液中,充分反应后分离出未反应的镁,然后在溶液中滴加1.0 mol•L-1的盐酸,加入盐酸的体积与得到沉淀的数据如下表:

实验次数加入盐酸的体积沉淀质量
160 mL0
280 mL0.78 g
3180 mL5.46 g
(1)开始沉淀时加入的盐酸体积必须大于             mL;若得到沉淀的量最大时,加入的盐酸体积为            mL。 

(2)合金中铝的质量分数为              

答案

(1)70(2分),150(2分)  (2)72 %(2分)

(1)根据表中数据可知,前60ml盐酸是中和过量的氢氧化钠的。0.78g沉淀是0.01mol,所以根据方程式H+AlO2+H2O=Al(OH)3↓可知,需要盐酸是0.01mol,其体积是10ml,因此开始沉淀时加入的盐酸体积必须大于80ml-10ml=10ml。当沉淀最大时,溶液中生成氯化钠是0.1L×1.5mol/L=0.15mol,需要盐酸是150ml。

(2)生成的氢氧化铝是(0.15L-0.07L)×1.0mol/L=0.08mol,则铝的质量是0.08mol×27g/mol=2.16g,其质量分数为

阅读理解

For thousands of years, man has enjoyed the taste of apples. Apples, which are about 85 percent water, grow almost everywhere in the world but the hottest and coldest areas (地区). Among the leading countries in apple production are China, France and the United States.

There are various kinds of apples, but a very few make up the majority of those grown for sale. The three most common kinds grown in the United States are Delicious, golden Delicious, and McIntosh.

 Apples are different in colour, size, and taste. The colour of the skin may be red, green, or yellow. They have various sizes, with Delicious apples being among the largest. The taste may be sweet or tart (酸的). Generally, sweet apples are eaten fresh while tart apples are used to make applesauce (苹果酱).

 Apple trees may grow as tall as twelve metres. They do best in areas that have very cold winters. Although no fruit is yielded during the winter, this cold period is good for the tree.

小题1:It can be learned from the text that Delicious apples are _______________

A.grown in France.

B.sold everywhere.

C.very big.      

D.quite sweet.小题2:Cold winter weather is good for _______________

A.the growth of apple trees.

B.producing large apples.

C.improving the taste of apples.

D.the increase of water in apples.小题3:China, France and the United States are considered to _______________

A.be large producers of apples.

B.be large producers of applesauce.

C.have the longest history in apple production.

D.have the coldest winter among apple producing countries.小题4:The word yielded in the last sentence means _______________

A.improved.

B.increased.

C.produced. 

D.sold.

单项选择题