Determine the ionization constant of a weak acid conductometrically.

 Determine the ionization constant of a weak acid (Say acetic acid) conductometrically.

Theory

Acetic acid is a weak acid and ionizes or dissociates to a small extent in an aqueous solution

 CH₃COOH ⇌ CH₃COO^- + H⁺

Its degree of ionization can be determined by conductivity measurement of its solutions of different strengths or normalities. Solutions of different concentrations are taken and their conductance is measured with the help of a conductivity bridge. Then the value of the specific conductance of each solution is calculated by the relation.

Specific conductance, K = cell constant x Measured conductance. The value of equivalent conductance can be calculated from specific conductance by the relation.

Equivalent conductance  ∧_v = K x V

at a particular dilution V, where V is the volume in ml. containing one gram equivalent of acetic acid. The equivalent conductance of acetic acid at infinite dilution A is noted from the tables of constants. 

Then the degree of ionization =   

Then ionization constant can be calculated by the relation

Requirements :

(a) Apparatus

Conductivity Bridge, conductivity cell, thermostat, and connecting wires

(b)Chemicals

250 ml N/10 acetic acid.

Chemical Equations

CH₃COOH ⇌ CH₃COO^- + H⁺

Procedure:

• Determine the cell constant i.e. cell constant = specific conductance/Observed conductance.
• Wash the electrodes of the conductivity cell with conductivity water and then with N/10 acetic acid solution.
• Place 50.0 ml of N/10 acetic acid solution in a beaker and immerse the conductivity cell in the solution. 
• Note the conductance of the solution from the direct reading conductivity bridge.
• Take 25 ml of N/10 acetic acid in a beaker with the help of a pipette and add to it 25 ml. of conductivity water to get N/20 acetic acid solution. Immerse the conductivity cell in N/20 acetic acid solution and note its conductance.
• Again take-25 ml of N/20 acetic acid and add to it 25 ml of conductivity water to get N/40 acetic acid. Immerse the conductivity cell in the acid solution and note its conductance.
• Repeat the process as in step 6 to get N/80 acetic acid and note its conductance
• Repeat step 7 to get M/160 acetic acid and N/320 acetic acid and note their conductance,
• Note the equivalent conductance of acetic acid at infinite dilution from the tables.

Observations and calculations

Equivalent conductance of acetic acid at infinite dilution at 25°C = 387 ohm-¹ cm-1 cm³ / g equivalent. 

Cell constant as determined (x) =...........cm-¹

Conductance of water used for the preparation of solutions ...... ohm-1      

­­­­­­	Observed conductance (ohm-1)	Specific conductance	Equivalent Constantance	Degree of ionisation	Ionisation constant
N/10 at v=10,000     N/20 at V=20,000   N/40 at v=40,000     N/80 at v=80,000         :         :         :         :         :         :         :         :         : N/320 at v=320,000	________         ________         ________           ________                         ________	K10         K20         K40           K80         :         :         :         :         :         :         :         :         :     K320

Result:

Dissociation constant of acetic acid = ______.

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