This is the 2478th most frequent Russian word.
председателем
Chairperson.
The word 'председателем' is used in instrumental case to indicate that the subject became a chairman after a certain action.
Он стал председателем клуба после вступления.
He became the chairman of the club after joining.
Here, 'председателем' identifies the role or position assigned during the election, again in instrumental case.
Председателем собрания был избран опытный юрист.
An experienced lawyer was elected as the chairman of the meeting.
In this sentence, 'председателем' is used in instrumental case, indicating the role associated with specific responsibilities.