Chloroacetic acid, HC2H2ClO2, has a greater acid strength than acetic acid, because the electronegative chlorine atom pulls electrons away from the O—H bond and thus weakens it. Calculate the hydronium-ion concentration of and the pH of a 0.0020 M solution of chloroacetic acid. Ka is 1.3 × 10–3.

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Alleei Alleei · Answer 1 · 2019-08-08T06:17:20+02:00

Answer : The hydronium ion concentration and the pH of the solution is, [tex] 1.08\times 10^{-3}M[/tex] and 2.97 respectively.

Solution : Given,

Concentration (c) = 0.0020 M

Acid dissociation constant = [tex]k_a=1.3\times 10^{-3}[/tex]

The equilibrium reaction for dissociation of [tex]CH_2ClCOOH[/tex] (weak acid) is,

[tex]CH_2ClCOOH\rightleftharpoons CH_2ClCOO^-+H^+[/tex]

initially conc. c 0 0

At eqm. [tex]c(1-\alpha)[/tex] [tex]c\alpha[/tex] [tex]c\alpha[/tex]

First we have to calculate the concentration of value of dissociation constant [tex](\alpha)[/tex].

Formula used :

[tex]k_a=\frac{(c\alpha)(c\alpha)}{c(1-\alpha)}[/tex]

Now put all the given values in this formula ,we get the value of dissociation constant [tex](\alpha}[/tex].

[tex]1.3\times 10^{-3}=\frac{(0.002\alpha)(0.002\alpha)}{0.002(1-\alpha)}[/tex]

By solving the terms, we get

[tex]\alpha=0.544[/tex]

Now we have to calculate the concentration of hydronium ion or hydrogen ion.

[tex][H^+]=c\alpha=0.002\times 0.544=1.08\times 10^{-3}M[/tex]

Now we have to calculate the pH.

[tex]pH=-\log [H^+][/tex]

[tex]pH=-\log (1.08\times 10^{-3})[/tex]

[tex]pH=2.97[/tex]

Therefore, the pH of the solution is, 2.97