## Chemistry: The Molecular Science (5th Edition)

$PH_3(aq) + H_2O(l) \lt -- \gt P{H_4}^+(aq) + OH^-(aq)$ $K_b = \frac{[OH^-][P{H_4}^+]}{[PH_3]}$
The ionization equation should describe the transfer of a proton from a water molecule to the base, therefore, the base and the water are the reactants, and the protonated base and a hydroxide ion are the products. The ionization constant expression is based on: $K_b = \frac{[Products]}{[Reactants]}$ * The water doesn't appear, because it is the solvent, and its concentration doesn't matter.