L3 - Equilibrium Law and the Strength of Acids and Bases

If you haven't realized it by now, it is possible to find the K_b value if you already know the K_a value, and vise versa.

The K_a and K_b equilibrium constants are related through the K_w, water equilibrium constant.  If we represent acids with an A, we can show which are the strong acids (HA) and which are the weak acids (A^-).  Then as they mix with water in a reversible reaction, we can balance the equations vertically (see right) to create an equation where water spontaneously decomposes into hydronium and hydroxide ions, this is the basis for the K_w equation.

As a result,

K_w = 1.0 x 10^-14 (ALWAYS)

And the equation relation between the different equilibrium constants is:

Video Example