For reaction `aA + bB harr cC + dD`, the Chemical Equilibrium Constant is represented by `K_a`. This relationship is built upon the activity of each component i in the reaction.
`hata _i` = the activity of component i = `hatf_i / dotf_i` where:
`hatf_i` = fugacity of component i and
`dotf_i` = fugacity of pure i in its standard state