At a certain temperature, the equilibrium constant $K_c$ for $A \rightleftharpoons B$ is $4$. If the initial concentration of $A$ is $1.0$ M (with $[B]_0 = 0$), find the equilibrium concentration of $B$.

Use an ICE table. Let $x$ = [B] at equilibrium. | | A | B | |---|---|---| | Initial | 1.0 | 0 | | Change | $-x$ | $+x$ | | Equilibrium | $1.0 - x$ | $x$ | Substitute into $K_c = \dfrac{[B]}{[A]} = 4$: $\dfrac{x}{1 - x} = 4 \Rightarrow x = 4 - 4x \Rightarrow 5x = 4 \Rightarrow x = 0.8$ M. Check: $[B]/[A] = 0.8 / 0.2 = 4$ ✓. The high $K_c$ means the equilibrium lies well to the right, which the answer $0.8$ M out of $1.0$ M confirms.