K a is a measure of the strength of the acid HX i.e., larger the value of K a , the stronger is the acid. K a is a dimensionless quantity with the understanding that the standard state concentration of all species is 1M. The values of the ionization constants of some selected weak acids are given in Table . .
Table . The pH of Some Common Substances Name of the Fluid pH Name of the Fluid pH Saturated solution of NaOH ~ Black Coffee . . M NaOH solution Tomato juice ~ .
Lime water . Soft drinks and vinegar ~ . Milk of magnesia Lemon juice ~ . Egg white, sea water .
Gastric juice ~ . Human blood . 1M HCl solution ~ Milk . Concentrated HCl ~– .
Human Saliva . The pH scale for the hydrogen ion concentration has been so useful that besides p K w , it has been extended to other species and quantities. Thus, we have: p K a = –log ( K a ) ( . ) Knowing the ionization constant, K a of an acid and its initial concentration, c, it is possible to calculate the equilibrium concentration of all species and also the degree of ionization of the acid and the pH of the solution.
A general step-wise approach can be adopted to evaluate the pH of the weak electrolyte as follows: Step . The species present before dissociation are identified as Brönsted-Lowry acids/bases. Step . Balanced equations for all possible reactions i.e., with a species acting both as acid as well as base are written.
Step . The reaction with the higher K a is identified as the primary reaction whilst the other is a subsidiary reaction. Step . Enlist in a tabular form the following values for each of the species in the primary reaction (a) Initial concentration, c.
(b) Change in concentration on proceeding to equilibrium in terms of α , degree of ionization. (c) Equilibrium concentration. Step . Substitute equilibrium concentrations into equilibrium constant equation for principal reaction and solve for α .
Step . Calculate the