01ICE + algebra
Write the ICE table with x, plug into K, solve. If K is very small (< 10^−4 vs. initial concentration), assume x is negligible to skip the quadratic.
Check the assumption: x should be < 5% of the initial value.
02Notebook box
Small K → 'initial − x ≈ initial'.
Worked example: A ⇌ 2B, K = 4 × 10^−6, [A]₀ = 1.0 → 4x² = 4 × 10^−6 → x = 1 × 10^−3 → [B] = 2 × 10^−3 M.