Question

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.

Answer 1

Answer : The hydronium ion concentration and the pH of the solution is,
`1.08* 10^(-3)M` and 2.97 respectively.

Solution : Given,

Concentration (c) = 0.0020 M

Acid dissociation constant =
`k_a=1.3* 10^(-3)`

The equilibrium reaction for dissociation of
`CH_2ClCOOH` (weak acid) is,

`CH_2ClCOOH\rightleftharpoons CH_2ClCOO^-+H^+`

initially conc. c 0 0

At eqm.
`c(1-\alpha)`
`c\alpha`
`c\alpha`

First we have to calculate the concentration of value of dissociation constant
`(\alpha)`.

Formula used :

`k_a=((c\alpha)(c\alpha))/(c(1-\alpha))`

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

`1.3* 10^(-3)=((0.002\alpha)(0.002\alpha))/(0.002(1-\alpha))`

By solving the terms, we get

`\alpha=0.544`

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

`[H^+]=c\alpha=0.002* 0.544=1.08* 10^(-3)M`

Now we have to calculate the pH.

`pH=-\log [H^+]`

`pH=-\log (1.08* 10^(-3))`

`pH=2.97`

Therefore, the pH of the solution is, 2.97