Which way will it go?

For aA + bB ⇌ cC + dD, concentrations are raised to their stoichiometric coefficients. Pure solids and pure liquids are omitted from the expression. K uses equilibrium concentrations; Q uses the concentrations at the moment being considered.

If Q < K, there is relatively too little product and the net reaction proceeds forward. If Q > K, it proceeds in reverse. If Q = K, the mixture is at equilibrium. A large K favours products; it says nothing by itself about speed.

For concentration calculations, first account for the reaction stoichiometry. An ICE table lists initial, change and equilibrium values. If a small-x approximation is used because K is small, state it and check that the neglected change is acceptably small.