Question

(5)

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 and the pH of a 2.49E-3 M solution of chloroacetic acid. K_a is 1.3E-3.

[H₃O⁺] = _____ M

pH = _____

Answer 1

Solution :-

Given data

HC₂H₂ClO₂ ­=2.49E-3 M

Ka = 1.3E-3

[H₃O⁺] = ? M

pH = ?

Lets first write the dissociation equation of the chloroacetic acid with water

HC₂H₂ClO₂ + H2O ------- > H3O⁺ + HC₂H₂ClO₂^-

Using the ka and the initial concentration we can calculate the [H3O+] and pH

Lets make the ICE table

         HC₂H₂ClO₂ + H2O ------- > H3O⁺ + HC₂H₂ClO₂^-

I        0.00249                                        0                  0

C          -x                                              +x              +x

E        0.00249 –x                                   x                 x

Now lets write the ka equation for the above reaction

Ka = [H3O⁺] [HC₂H₂ClO₂^-] / [HC₂H₂ClO₂]

Now lets put the values in the formula and calculate the value of x

1.3E-3 = [x][x]/[0.00249-x]

1.3E-3 * 0.0.00249-x = x²

3.237E-6 - 1.3E-3x =x²

x²+1.3E-3 x -3.237E-6 =0

by solving this using quadratic equation we get

x = 0.00126

Therefore concentration of hydronium ion [H3O+] = 1.23E-3 M

Now lets calculate the pH

pH = -log[H3O+]

pH= - log [1.23E-3]

pH = 2.91