Question

2,4-Dichlorophenoxyacetic acid ("2,4-D") is a widely used herbicide with the molecular formula C8H6Cl2O3. The pKa of 2,4-D is 2.73. A certain water-based herbicide product contains 110 gL−1 of 2,4-D. What is the pH of the 2,4-D solution?

Answer 1

From the given mass of acid, its moles are calculate. Given pKa value is used to calculate ka of acid. Equilibrium reaction of acid with water can be used to find Hydronium ion concentration.

Calculate moles of acid

Molar mass is 221.0384 g/mol

Mol = 110 g x 1 mol / 221.0384 g = 0.4977

[HC8H5Cl2O3] = 0.4977 mol / 1 L = 0.498 M

ICE

      HC8H5Cl2O3(aq)    + H2O (l)     --- > C8H5Cl2O3-(aq)     + H3O+(aq)

I      0.498                                                          0                              0

C       -x                                                            +x                              +x

E    (0.498 -x)                                                     x                                  x

Ka=10^(-pka) = 10^(-2.73)=0.00186

Ka = 0.00186 = x^2 / ( 0.498-x)

(0.498-x) * 0.00186 = x^2

(0.498*0.00186)-0.00186 x = x^2

Lets solve this quadratic equation.

x= 0.0295 = [H3O+]

pH = -log[H3O+]= - log ( 0.0295) = 1.53

pH of this acid solution is 1.53