No2 and n2o4 undergo the reaction shown. when a sealed
container of no2 reaches chemical equilibrium, which must be true?
a) no n2o4 is present.
b) no chemical reactions are occurring.
c) the rates of the forward and reverse reactions are equal.
d) the maximum number of molecules has been reached.

Explanation:

NO₂ and N₂O₄ undergo the following equilibrium reaction:

That is a reversible reaction, i.e. there are two simultaneous reactions: the direct or forward reaction and the reverse reaction:

At the beginning, only NO₂(g) is in the sealed container. The NO₂ concentration is maximum, and the rate of the forward reaction is maximum.

As the reaction progresses, the concentration of NO₂ diminishes, and, consequently, the rate of the forward reaction decreases.

As soon as the N₂O₄ appears, the reverse reaction starts. At the beginning the rate is low, but as the N₂O₄ concentration increases the rate of the reverse reaction increases.

When both forward and reverse rates become equal the equilibrium has been reached. This is what is called a dynamical equilibrium.

Then, as per the choices, you have that, at equilibrium:

A) No N₂O₄ is present:

B) No chemical reactions are occurring.

C) The rates of the forward and reverse reactions are equal.

D) The maximum number of molecules has been reached.

where are the choice options?