Which two factors must be equal when a chemical reaction reaches equilibrium?
(1) the concentration of the reactants and the concentration of the products
(2) the number of reactant particles and the number of product particles
(3) the rate of the forward reaction and the rate of the reverse reaction
(4) the mass of the reactants and the mass of the products

option (3) the rate of the forward reaction and the rate of the reverse reaction.

Explanation:

1) Chemical equilibrium is said to be a dynamic equlibrium, not static.

2) A static equilibrium is when nothing changes at all, but a dynamic equilibrium is reached when the rate of change in one direction is equal to the rate of change in the opposite direction, and so there is not a net change, the net change is zero:.

For example when in a room the number of people that comes in is equal to the number of people that comes out, then the number of people in the room does not change.

3) In a reversbile reaction, the reactant molecules still react (the forward reaction) to form new product molecules, but at the same time the product molecules react (the reverse reaction) exactly in the opposite sense to produce new molecutles of the same reactants.

If the rate at which the both reverse and forward reactions is the same, then the concentrations of both reactants and products do not change, i.e. there is not a net change, and the chemical equilibrium has been reached.

That is what the chemical equilibrium is: the state at which the rate of the forward reaction is equal to the rate of the reverse reaction, with the consequent equilibrium of the concentrations.