Determination of the 25 Pre-lab Questions Dissociation Constant of a weak Acid B

Question

Determination of the 25 Pre-lab Questions Dissociation Constant of a weak Acid Before beginning this experiment in the laboratory, you should be able to answer the following questions. I. Define Bransted-Lowry acids and bases. 2. Differentiate between the dissociation constant and equilibrium constant for the dissociation of a weak acid, HA, in aqueous solution. 3. Why isn't the pH at the equivalence point always equal to 7 in a neutralization titration? When is it 7? 4. What is the pKa of an acid whose K. is 6.5x10 6? 5. Why must two electrodes be used to make an electrical measurement such as pH? 6. What is a buffer solution? Copyright O 2012 Pearson Education, Inc.

Explanation / Answer

Q1.

First, let us define Bronsted Lowry acid/base:

Bronsted Lowry acid: any species that will donate H+ (protons) in solution, and makes pH lower (i.e HCl)

Bronsted Lowry base: any species that will accept H+ (protons) in solution, and makes pH higher (NH3 will accept H+ to form NH4+)

Typically, acid/bases are shown in the left (reactants)

when we write the products:

Bronsted Lowery conjugate base = the base formed when the B.L. acid donates its H+ proton ( i.e. HCl -> Cl-

Bronsted Lowery conjugate acid = the acid formed when the B.L. base accept its H+ proton ( i.e. NH4+ has accept H+ proton)

Note that, typically conjugate bases/acids are shown in the right (product) side

Q2.

Dissociation constant --> Ka

Equilibrium constant --> K

Note that K must include all vlaues, it typically includes the dissociation of water

[H2O] = [H+][OH-]

whereas Ka includes it in a numerical way

i.e.

Ka = K*Kw

Q3.

When weak acid or bases are titrated, they will ionize in solution to form:

HA --> H+ and A- in equilibrium

A- is present in neutralization

A- + H2O <-> HA + OH- will form

OH- is basic, so the weak acid after neutralization will form a basic solution

note that the true is for bases, they will show a slightly acidic pH after neutralization

ONLY in strong acid/bases you will expect a pH = 7

Q4.

pKa = -log(Ka)

by definition

pKa = -log(6.5*10^-6)

pKa = 5.187

Q5

We need conductivity, so two electrodes will help us to make those electrons to flow from negative to positive

Q6.

Buffer are solutions made up with conjguate acid + weak base or vice verse, conjugate base + weak acid

they will sustain H+ or OH- addition and pH will not change drastically

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