CHEMISTRY COMMUNITY

Is the equilibrium constant K the ratio of the initial or equilibrium concentrations of the products and reactants?

It is a ratio of the equilibrium concentrations.

Since K is the equilibrium expression, you use the concentrations at equilibrium. The initial concentrations can be used to do other calculations like with weak acids and bases.

K is the equilibrium constant so the numbers you use are the equilibrium concentrations of the products and reactants. This tells you that this ratio will be approached when a reaction reaches equilibrium from its initial concentrations.

Q would be anything other than equilibrium, and K is the equilibrium.

Adding on, K is referring to the concentrations held to the specific power and ratio as stated in the balanced equilibrium equation given in the problem. Knowing the concentrations won't do you any good if they aren't held to the correct power within the fraction for K.

It also matters if it's Kc or Kp. While Kc does refer to the ratio of concentrations to their respective powers of products and reactants, Kp will refer to the ratio of partial pressures to their respective powers of products and reactants.