diff --git a/docs/sch4uz.md b/docs/sch4uz.md
index 0c9e7a0..bf3abff 100644
--- a/docs/sch4uz.md
+++ b/docs/sch4uz.md
@@ -735,6 +735,7 @@ $$K_b = \ce{\frac{[HX+][OH-]}{[X]}}$$
 
 At all temperatures:
 $$K_a \times K_b = K_w$$
+$$pK_a + pK_b = pK_w$$
 
 ### Acid strength