JEE Main 2021 · Chemistry · Redox Reactions and Titrations · medium

Sodium thiosulphate pentahydrate (Na2S2O3·5H2O, molar mass 248 g mol^-1), 12.4 g, is completely oxidised to sulphate by chlorine in aqueous alkaline…

Numerical answer

Sodium thiosulphate pentahydrate (

\mathrm{Na_2S_2O_3\cdot5H_2O}

, molar mass

248\ \mathrm{g\,mol^{-1}}

12.4\ \mathrm{g}

, is completely oxidised to sulphate by chlorine in aqueous alkaline medium (chlorine is reduced to chloride). The mass of

\mathrm{Cl_2}

consumed, in grams to the nearest integer, is (Cl = 35.5)

Show answer and solution

Answer: 14

Moles of salt

=12.4/248=0.05

. In

\mathrm{S_2O_3^{2-}}

each S has oxidation state

+2

and goes to

+6

\mathrm{SO_4^{2-}}

, so two S atoms lose

2\times4=8

electrons per ion. Each

\mathrm{Cl_2}

gains

2

electrons. Reaction:

\mathrm{S_2O_3^{2-}}+4\mathrm{Cl_2}+10\mathrm{OH^-}\to2\mathrm{SO_4^{2-}}+8\mathrm{Cl^-}+5\mathrm{H_2O}

. Moles of

\mathrm{Cl_2}=4\times0.05=0.20

, mass

=0.20\times71=14.2\ \mathrm{g}\approx14\ \mathrm{g}

. (Iodine oxidises thiosulphate only to tetrathionate, needing only

0.025

mol

\mathrm{I_2}

, which is why the oxidant matters.)

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