JEE Challenger
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ChemMedAdv 2007 • Paper 1

Calculate Total Moles of Chlorine Gas Evolved During NaCl Electrolysis

Comprehension Passage

Chemical reactions involve interaction of atoms and molecules. A large number of atoms/molecules (approximately 6.023×10236.023 \times 10^{23}) are present in a few grams of any chemical compound varying with their atomic/molecular masses. To handle such large numbers conveniently, the mole concept was introduced. This concept has implications in diverse areas such as analytical chemistry, biochemistry, electrochemistry and radiochemistry. The following example illustrates a typical case, involving chemical/electrochemical reaction, which requires a clear understanding of the mole concept.

A 4.0 molar4.0\text{ molar} aqueous solution of NaCl\text{NaCl} is prepared and 500 mL500\text{ mL} of this solution is electrolysed. This leads to the evolution of chlorine gas at one of the electrodes (atomic mass : Na=23,Hg=200\text{Na} = 23, \text{Hg} = 200; 1 Faraday=96500 coulombs1\text{ Faraday} = 96500\text{ coulombs}).

The total number of moles of chlorine gas evolved is

Options

A

0.50.5

B

1.01.0

Correct
C

2.02.0

D

3.03.0

Topics & Concepts

ElectrochemistryMole ConceptElectrolysis

Step-by-Step Solution

To determine the total number of moles of chlorine gas evolved during the electrolysis of the NaCl\text{NaCl} solution, we perform the following steps:

  1. Calculate the initial moles of NaCl\text{NaCl} present in the solution:
    Moles of NaCl=Molarity×Volume (in L)\text{Moles of NaCl} = \text{Molarity} \times \text{Volume (in L)}
    Moles of NaCl=4.0 M×0.500 L=2.0 moles\text{Moles of NaCl} = 4.0 \text{ M} \times 0.500 \text{ L} = 2.0 \text{ moles}

  2. Determine the moles of chloride ions (Cl−\text{Cl}^-):
    Since NaCl\text{NaCl} completely dissociates in an aqueous solution:
    NaCl(aq)→Na+(aq)+Cl−(aq)\text{NaCl(aq)} \rightarrow \text{Na}^+(\text{aq}) + \text{Cl}^-(\text{aq})
    Moles of Cl−=2.0 moles\text{Moles of Cl}^- = 2.0 \text{ moles}

  3. Determine the moles of chlorine gas (Cl2\text{Cl}_2) evolved at the anode:
    The oxidation reaction taking place at the anode is:
    2Cl−(aq)→Cl2(g)+2e−2\text{Cl}^-(\text{aq}) \rightarrow \text{Cl}_2(\text{g}) + 2e^-

    From the stoichiometry of the reaction, 2 moles2 \text{ moles} of Cl−\text{Cl}^- ions yield 1 mole1 \text{ mole} of Cl2\text{Cl}_2 gas.

    Moles of Cl2 gas=Moles of Cl−2=2.02=1.0 mole\text{Moles of Cl}_2 \text{ gas} = \frac{\text{Moles of Cl}^-}{2} = \frac{2.0}{2} = 1.0 \text{ mole}

Thus, the total number of moles of chlorine gas evolved is 1.01.0.

Correct Option: (B)

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