Michael Faraday was the first scientist who described the quantitative aspects of electrolysis. Now Faraday’s laws also flow from what has been discussed earlier. Faraday’s Laws of Electrolysis After his extensive investigations on electrolysis of solutions and melts of electrolytes, Faraday published his results during 1833-34 in the form of the following well known Faraday’s two laws of electrolysis:
(i) First Law: The amount of chemical reaction which occurs at any electrode during electrolysis by a current is proportional to the quantity of electricity passed through the electrolyte (solution or melt).
(ii) Second Law: The amounts of different substances liberated by the same quantity of electricity passing through the electrolytic solution are proportional to their chemical equivalent weights (Atomic Mass of Metal ÷ Number of electrons required to reduce the cation).
Consider the following statements about the quantity of electricity in electrolysis.
(A) The charge on one mole of electrons, called one Faraday, is obtained by multiplying the charge on one electron, 1.6021 × 10−19 C, by the Avogadro constant.
(B) In the relation Q = It, the quantity of electricity Q is in coulombs when the current I is in ampere and the time t is in minutes.
(C) One mole of Mg2+ ions requires 2 F of electricity for its reduction to magnesium metal.
(D) A coulometer is a standard galvanic cell that was used to determine the quantity of electricity passed.
Which of the statements given above are correct? Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → (A) and (C) only
The quantity of electricity passed in electrolysis is found from the current and the time, Q = It. The charge on one mole of electrons is the Faraday (F), and the number of faradays needed depends on the electrons in the electrode reaction.
(A) The charge on one mole of electrons, called one Faraday, is obtained by multiplying the charge on one electron, 1.6021 × 10−19 C, by the Avogadro constant. — True. NA × 1.6021 × 10−19 C = 6.02 × 1023 mol−1 × 1.6021 × 10−19 C = 96487 C mol−1, which is 1 F.
(B) In the relation Q = It, the quantity of electricity Q is in coulombs when the current I is in ampere and the time t is in minutes. — False. Q comes out in coulombs only when t is in seconds (1 C = 1 A × 1 s).
(C) One mole of Mg2+ ions requires 2 F of electricity for its reduction to magnesium metal. — True. Each Mg2+ ion takes two electrons, so one mole needs 2 mol of electrons:
\( \displaystyle \ce{Mg^2+(l) + 2e^- -> Mg(s)} \)
(D) A coulometer is a standard galvanic cell that was used to determine the quantity of electricity passed. — False. A coulometer is a standard electrolytic cell. The charge passed was found from the amount of metal (generally silver or copper) deposited or consumed in it.
NCERT (2026-27 edition):
2.5 Electrolytic Cells and Electrolysis – “There were no constant current sources available during Faraday’s times. The general practice was to put a coulometer (a standard electrolytic cell) for determining the quantity of electricity passed from the amount of metal (generally silver or copper) deposited or consumed. However, coulometers are now obsolete and we now have constant current (I) sources available and the quantity of electricity Q, passed is given by Q = It. Q is in coulombs when I is in ampere and t is in second.”
“We know that charge on one electron is equal to 1.6021 × 10–19 C. Therefore, the charge on one mole of electrons is equal to: NA × 1.6021 × 10–19 C = 6.02 × 1023 mol–1 × 1.6021 × 10–19 C = 96487 C mol–1. This quantity of electricity is called Faraday and is represented by the symbol F.”
“…It is obvious that one mole of Mg2+ and Al3+ require 2 mol of electrons (2F) and 3 mol of electrons (3F) respectively.”