The ionisation constant of an acid, Ka, is the measure of the strength of an acid. The Ka values of acetic acid, hypochlorous acid and formic acid are 1.74 × 10-5, 3.0 × 10-8 and 1.8 × 10-4 respectively. Which of the following orders of pH of 0.1 mol dm–3 solutions of these acids is correct?


o For a weak acid HX (partial ionization) the Equilibrium can be expressed as :


HX (aq.) + H2O (l) H3O+ (aq.) + X (aq.)


The concentration of this H3O+ can be termed down as [H3O+] = √(Ka. C),


where Ka = ionization constant of the acid and C = the initial molar concentration of the acid.


Hence, for the same concentration [H3O+] Ka


But pH=-log[H3O+]


So, larger the value of Ka, larger will be the value of [H3O+]and obviously lower the value of pH.


From the given The Ka values of acetic acid, hypochlorous acid and formic acid are 1.74 × 10-5, 3.0 × 10-8 and 1.8 × 10-4 respectively the order of increasing Ka is :


acetic acid <formic acid <hypochlorous acid


hence the order of pH of 0.1 mol dm–3 solutions of these acids is


(iv) formic acid > acetic acid > hypochlorous acid as in accordance with the Ka values (larger Ka means lower pH).

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