Question

Which statement is true of a reversible reaction at equilibrium?

a) The concentration of reactants is less than the concentration of products.
b) The concentration of reactants and the concentration of products are equal.
c)The concentration of reactants is greater than the concentration of products.
d) The concentration of reactants and the concentration of products are constant.
e) The concentration of reactants is decreasing and the concentration of products is increasing.

Answer 1

The concentrations must be constant, because we have no idea about the reaction quotient K of what it's value is which tells us where the equilibrium lies.
Say if K>1 then we know that the equilibrium favors products and the concentration of products must be greater. The opposite must be true if K<1

So, Without this information we can't say anything about whether the reactants or products are at a higher concentration.

Instead, all we know is the general rule that defines equilibrium a state where the concentrations of both reactants and products are constant, implying that the foreword and reverse reactions occur at the same rate.