A chemical equilibrium is a chemical reaction that is reacting in both directions (forward and backwards) at the same time, and at the same rate. Nothing appears to change (macroscopically - on the large scale), but both reactions are dynamic microscopically.
Understanding how a chemical equilibrium functions is most important. If you can understand that the equilibrium will respond, whenever possible, to oppose any changes that are introduced, then you can work out the answer to any question by thinking it thr
The application of numbers to equilibrium is relatively straightforward, but only if you are confident with mole calculations (Stoichiometric relationships), and you also have a sound understanding of how an equilibrium functions (Equilibrium).