Question

What is the purpose of the experiements determining the dissociation constant of a weak acid using ph measurement and monitoring acid-base titrations with a ph meter? We combined both experiements together into one, just not understand the purpose of the experiment.

Answer 1

The main purpose of experiements determining the dissociation constant to identify an unknown weak acid by titration with a standard sodium hydroxide solution and monitoring the pH of the titration solution using a pH meter.

once the measurement is done , then plot pH vs volume of NaOH than you will be able to determine the equivalent molecular weight and dissociation constant (Ka) of the unknown acid. These obatined values will then let you decide the identity of the weak acid.

if weak acid, HA, is dissolved in water, only some of the molecules dissociate to yield H⁺ and A^- ions.

dynamic equilibrium is obtained at this point:

HA H+ + A-

Under these equilibrium conditions, the total concentration of each species remains constant, even if the species in solution are constantly dissociating and recombining.

Therefore degree of dissociation of the weak acid is used to characterize the acid, and is calculated according to the equation:

Ka = [H+] [A-] / [HA] , where Ka is the Acid Constant of the unknown acid

pKa = - Log (Ka)

The pKa of the acid can be determined by determining the pH value at 1/2 VNaOH, which can also be estimated from the titration curve.

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