concentration of species in principal reaction. Step . Calculate pH = – log[H O + ] The above mentioned methodology has been elucidated in the following examples. The following proton transfer reactions are possible: ) HF + H O H O + + F – K a = .
× – ) H O + H O H O + + OH – K w = . × – As K a >> K w , [ ] is the principle reaction. HF + H O H O + + F – Initial concentration (M) . Change (M) – .
α + . α + . α Equilibrium concentration (M) . – .
α . α . α Substituting equilibrium concentrations in the equilibrium reaction for principal reaction gives: K a = ( . α ) / ( .
– . α ) = . α / ( – α ) = . × – We obtain the following quadratic equation: α + .
× – α – . × – = The quadratic equation in α can be solved and the two values of the roots are: α = + . and – . The negative root is not acceptable and hence, α = .
This means that the degree of ionization, α = . , then equilibrium concentrations of other species viz., HF, F – and H O + are given by: [H O + ] = [F – ] = c α = . × . = .
× – ) = . Problem . The pH of .1M monobasic acid is . .
Calculate the concentration of species H + , A – Problem . The ionization constant of HF is . × – . Calculate the degree of dissociation of HF in its .
M solution. Calculate the concentration of all