问题 实验题

草酸合铁(Ⅲ)酸钾晶体Kx[Fey(C2O4)z]·nH2O可用于摄影和蓝色印刷。为确定其组成,某化学兴趣小组在没有氧气干扰下进行如下实验:

实验Ⅰ.草酸根含量的测定

称取0.9820 g样品放入锥形瓶中,加入适量稀硫酸和一定量水,加热至343~358 K,用0.2000

mol/LKMnO4溶液滴定,滴定过程中草酸根离子转变成CO2逸出反应容器,KMnO4转变成Mn2。达到滴定终点时,共消耗KMnO4溶液12.00 ml。

实验Ⅱ.铁含量的测定

将上述滴定过草酸根的保留液充分加热,使淡紫红色消失。冷却后,调节溶液pH并加入过量的KI固体,充分反应后调节pH至7左右,滴入几滴淀粉溶液做指示剂,用0.2500mol·L-1 Na2S2O3标准溶液滴定至终点,共消耗Na2S2O3溶液8.00 ml。(已知:实验Ⅱ中涉及的部分离子方程式为:I2+2S2O32=2I+S4O62

请根据上述实验回答下列问题

(1)实验Ⅰ在滴定过程中反应的离子方程式为                               

(2)MnO4在酸性条件下,加热能分解为O2,同时生成Mn2。若实验Ⅱ中没有对实验Ⅰ的保留液进

行充分加热,则测定的Fe3的含量将会        (填“偏高”、“偏低” 或“不变”)。

(3)实验Ⅱ用淀粉溶液做指示剂,则滴定终点观察到的现象为                 

(4)通过计算确定样品晶体的组成(写出计算过程)。

答案

(1)2MnO4+5H2C2O4+6H=2Mn2+8H2O+10CO2↑(2分)

(2)偏高(2分)

(3)滴下最后一滴Na2S2O3标准溶液,溶液由蓝色变成无色,且半分钟内不变色(2分)

(4)(6分)

2MnO4- ~ ~~~~5C2O42-                  2S2O32~ I2~ 2 Fe3+

2                 5                       2             2

0.2×12×10-3mol    n(C2O42)           0.25×8×10-3mol    n(Fe3+)

解得n(C2O42) = 6×10-3mol  (1分)        n(Fe3+) = 2×10-3mol (1分)

根据电荷守恒原理:n(K+) =6×10-3mol    (1分)

根据质量守恒原理m(H2O) =0.9820-39×6×10-3-56×2×10-3-88×6×10-3=0.108g

n(H2O)=6×10-3 mol    (1分)

n(K+):n(Fe3+):n(C2O42):n(H2O)=3:1:3:3  (1分)

所以化学式为:K3Fe(C2O4)3·3H2O    (1分)

题目分析:(1)高锰酸钾具有强氧化性,能把草酸氧化生成CO2,而还原产物是Mn2,反应的化学方程式为2MnO4+5H2C2O4+6H=2Mn2+8H2O+10CO2↑。

(2)若实验Ⅱ中没有对实验Ⅰ的保留液进行充分加热,则溶液中有剩余的MnO4,MnO4能氧化I生成单质碘,从而导致与单质碘反应的Na2S2O3溶液的体积增加,因此使测定的Fe3的含量将会偏高。

(3)由于单质碘遇淀粉显蓝色,所以终点时的实验现象是滴下最后一滴Na2S2O3标准溶液,溶液由蓝色变成无色,且半分钟内不变色。

(4)根据题意并结合有关的反应方程式可知

2MnO4- ~ ~~~~5C2O42-                  2S2O32~ I2~ 2 Fe3+

2                 5                      2             2

0.2×12×10-3mol    n(C2O42)           0.25×8×10-3mol    n(Fe3+)

解得n(C2O42) = 6×10-3mol            n(Fe3+) = 2×10-3mol

根据电荷守恒原理:n(K+) =6×10-3mol

根据质量守恒原理m(H2O) =0.9820-39×6×10-3-56×2×10-3-88×6×10-3=0.108g

n(H2O)=6×10-3 mol

n(K+):n(Fe3+):n(C2O42):n(H2O)=3:1:3:3

所以化学式为:K3Fe(C2O4)3·3H2O

单项选择题
单项选择题

A narrowing of your work interests is implied in almost any transition from a study environment to managerial or professional work. In the humanities and social sciences you will at best reuse only a fraction of the material (1) in three or four years’ study. In most career paths academic knowledge only (2) a background to much more applied decision-making. Even with a " training " form of degree, (3) a few of the procedures or methods (4) in your studies are likely to be continuously relevant in your work. Partly this (5) the greater specialization of most work tasks compared (6) studying. Many graduates are not (7) with the variety involved in (8) from degree study in at least four or five subjects a year to very standardized job (9) . Academic work values (10) inventiveness, originality, and the cultivation of self-realization and self-development. Emphasis is placed (11) generating new ideas and knowledge, assembling (12) information to make a " rational " decision, appreciating basic (13) and theories, and getting involved in fundamental controversies and debates. The humanistic values of higher (14) encourages the feeling of being (15) in a process with a self-developmental rhythm. (16) , even if your employers pursue enlightened personnel development (17) and invest heavily in " human capital " —for example, by rotating graduate trainees to (18) their work experiences—you are still likely to notice and feel (19) about some major restrictions of your (20) and activities compared with a study environment.

19()

A.strange

B.ashamed

C.funny

D.sorry