Q8 of 54 Page 86

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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