①称量绿矾样品，配成100mL待测溶液，

②取一定体积待测液置于锥形瓶中，并加入一定量的硫酸，

③将标准浓度的KMnO₄溶液装入滴定管中，调节液面至a mL处，

④滴定待测液至滴定终点时，滴定管的液面读数b mL，

⑤重复滴定2～3次．

和 一样重。

    Long long ago, there was a swan（天鹅）with golden feathers（金羽毛）.She lived in a lake.

    A woman lived in a small house1the lake with her two daughters. They were very poor. They worked hard all year round,2 still, they lived a hard life and sometimes they even didn't have enough 3to buy food.

    The swan was 4to see that. She said to herself, “I'll give one of my5 to them each day, then they can live a happy life with the money selling my feathers.” That evening, she6to the poor woman's house and left a golden feather on the table without saying7.From then on, the swam came every day and gave them a feather. The woman was very happy because their life was much8than before.

    But day after day, the woman became greedy（贪婪）.She said to her 9”The swan may fly away one day. If so, we will be poor again. We should take all10feathers when she comes next time.” “Oh, no, Mom!” cried the daughters, “This will 11the swan. She helps us a lot !”But the mother wouldn't listen. When the 12came as usual, the mother caught her and took all her feathers .But suddenly, the golden feathers 13chicken feathers.

    Then ,the Golden Swan said, “Poor Mother, I came to 14 you, but you wanted to kill me. Now, I am leaving and will 15come back. Never be greedy!” With these words, the swan flew away.