K_a = \frac{{\left[ {H^ + } \right]\left[ {A^ - } \right]}}{{\left[ {HA} \right]}}
% MathType!MTEF!2!1!+-
% feaafaart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn
% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqr1ngB
% PrgifHhDYfgasaacH8YjY-vipgYlh9vqqj-hEeeu0xXdbba9frFj0-
% OqFfea0dXdd9vqai-hGuQ8kuc9pgc9q8qqaq-dir-f0-yqaiVgFr0x
% fr-xfr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaam4sam
% aaBaaaleaacaWGHbaabeaakiabg2da9maalaaabaWaamWaaeaacaWG
% ibWaaWbaaSqabeaacqGHRaWkaaaakiaawUfacaGLDbaadaWadaqaai
% aadgeadaahaaWcbeqaaiabgkHiTaaaaOGaay5waiaaw2faaaqaamaa
% dmaabaGaamisaiaadgeaaiaawUfacaGLDbaaaaaaaa!45D0!
Pasting the above "text" into MathType 5.0 will enter the above equation and make it possible to edit, save, and use in applications such as MS Word.