Keq, Kw, Kd, Ka, pKa?

They are all equilibrium constants. Equilibrium constants are ratios between the ions and molecules in solutions. Salts, strong acids and bases dissolve completely to form ions. Weak acids and bases form very few ions and most of the compound remains a neutral molecule. A weak acid might form ions as follows:

HA -> H(+) + A(-)

The equilibrium constant is

Keq = {H(+)][A(-)]/[HA}

All concentrations are exprerssed in moles/liter. Equilibrium constants become important when calculating the pH of weak acids and bases. If M is the molarity of the solution, the Keq becomes:

Keq = X^2/(M - X)

X is the concentrration of either H(+) or A(-). The pH is calculated from the concentration of the hydrogen ions.

keq=constant at equilibrium

kw=constant for water

kd=constant for dissociation

ka=constant for acid

pKa=constant pressure

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