In an experiment 0.04 F was passed through 400ml of a 1m solutionof NaCl. what would be the pH  of the solution after electrolysis ?

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llltkl | College Teacher | (Level 3) Valedictorian

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NaCl is a strong electrolyte, while H2O is a weak electrolyte. In moderately concentrated solution (1M), the following ions will prevail:

NaCl breaks down into Na+ and Cl- ions

H2O breaks down into H+ and OH- ions.

When electric current is passed through the solution, both Na+ and H+ ions move towards the cathode. But Na+ being more electropositive, exhibits lesser tendency to gain electrons, hence H+ ions will be preferntially discharged at the cathode

H+ + e gives  H; H+H gives  H2(stable, gas)

Similarly, both OH- and Cl- ions will move towards the anode. Although OH- has slightly lower discharge potential, owing to higher concentration of Cl- ions, they will be preferntially discharged at the anode

Cl- - e gives  Cl; Cl + Cl gives  Cl2(stable, gas)

Due to the discharge of H+ ions, the concentration of OH- ions keep on increasing and the pH rises. The exact amount can be calculated from Farady's conditions which states that for a monovalent ion, one Faraday current discharges one gram-ions of it.

Here 0.04 F should discharge 0.04 gram-ions of H+

Hence, [OH]- = 0.04 M

or, pOH = - log 0.04 

            =1.40

hence pH = 14-1.40 = 12.60 (as pH + pOH = 14 at 25 degree Celsius, for aqueous systems).

 

 

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