John was elected chairman of the students' union.

解答如下：

表示独立无二的职位/头衔前面前使用零冠词，即不用任何冠词。

The teacher appointed a safe student as monitor of our class.  

老师任命了一个谨小慎微的学生担任我们的班长.

He was proud to be elected chairman of the Students'Union.  

他为当选学生会主席而自豪.

Whether he will be elected as chairman is still disputed.  

他是否当选为主席,仍然有争论.

We made him monitor of our class ．

我们让他当我们班班长.

He is President of his own self-proclaimed republic.  

他是其自己宣布成立的共和国的总统。