Before beginning this experiment in the laboratory, you should be able to answer

Question

Before beginning this experiment in the laboratory, you should be able to answer the following questions. Define Bronsted-Lowry acid* and bases. Differentiate between live dissociation constant and equilibrium constant for the dissociation of a weak acid. HA, in aqueous solution. Why isn't the pH at the equivalence point always equal to 7 in a neutralization titration? When is it 7? What is the pK_a of an acid whose K_a is 6.5 times 10^-6? Why must two electrodes be used to make an electrical measurement such as pH? What is a buffer solution?

Explanation / Answer

1.J.N. Bronsted and T.M. Lowry independently and simultaneously(1923) put forward a more general concept for acids and bases.According tot his concept:

(i) Acid is a substance (molecule or ion) which has a tendency to donate a proton to any substance,and

(ii) A base is a substance (molecule or ion) which has a tendency to accept aq proton from any other substance.

In other words,an acid is a proton donor and base is a proton acceptor.Infact ,acid-base reaction involves tranference of proton.That is why,this concept is also called proton transfer theory of acids and bases.

2)  The dissociation of a weak acid, HA, in aqueous solution is demonstrated by the following equation-

HA + H2O (l) H+ (aq) + A- (aq) (Remember that H+ can be used to represent H3O+)
The equilibrium constant for this reaction is given by Keq = ([H+][A-])/([HA]),

where [H+] and[ A-] are the product concentrations, HA is the reactant concentration, and water is assumed to be a pure liquid and thus not included in the equilibrium expression.
Similarly, the acid dissociation constant for this expression is Ka = ([H+][A-])/([HA])

where [H+] and [A-] are the ion concentrations yielded by the original weak acid [HA]. It is a measure of the degree to which the weak acid dissociates into its respective ions at equilibrium.
From both expressions, it can be concluded that, in this specific case, the dissociation constant and equilibrium constant for the dissociation of a weak acid are the same.

3) The pH at the equivalence point is not always equal to 7. In a strong base-weak acid neutralization titration, the weak acid yields only small amounts of protons which are immediately neutralized by the base. Also, the strong conjugate base of the weak acid is also produced in the process, making the solution more basic.

4) pKa = - log Ka
= - log (6.5 x 10-6)
= -(0.81-6) =5.19

5) Two electrodes must be used to make an electrical measurement such as pH. One electrode is utilized to sense the H3O+ concentrations in the solution while the other is used to develop a known potential that is to be used as a reference.

6) A buffer solution is one which can resist change in its pH on the addition of an acid or a base.

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