Question

# The equivalent mass of sodium thiosulphate ($Na_2 S_2 O_3.5H_2 O$) in the reaction, $2Na_{ 2 }S_{ 2 }O_{ 3 }\quad +\quad I_{ 2 }\quad →\quad 2NaI\quad +\quad Na_{ 2 }S_{ 4 }O_{ 6 }$ is A- 248B- 124C- 596D- 62

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Hint: Equivalent mass calculation will be different for different types of substances. For substances involving in reduction or oxidation equivalent mass will be the mass that involves the transfer of one mole of electrons. Identify the element undergoing oxidation or reduction in the given compound.

We are asked to find the equivalent mass of sodium thiosulphate. We will calculate the number of equivalents in one mole of sodium thiosulphate. In the reaction except sodium and iodine remaining elements oxidation state remains the same. Let the oxidation state of sulphur be x and y in the left hand side and right-hand side of the reaction respectively.
Let us first calculate x. The total charge on the compound is 0. The oxidation state of oxygen is -2 and sodium is +1.
$2\times 1\quad +\quad 2\times x\quad +\quad 3\times -2\quad =\quad 0$
$2x\quad =\quad +4$
The oxidation state of sulphur is +2 in sodium thiosulphate.
Let us calculate y. The total charge on the compound is 0.
$2\times 1\quad +\quad 4\times y\quad +\quad 6\times -2\quad =\quad 0$
$4y\quad =\quad 10$
$y\quad =\quad +\frac { 5 }{ 2 }$
Change in the oxidation state of sulphur is $\frac { 1 }{ 2 }$.
As sodium thiosulphate contains 2 sulphur atoms electrons lost by each mole of sodium thiosulphate will be $2\times \frac { 1 }{ 2 } \quad =\quad 1$.Therefore, each mole of sodium thiosulphate involves in 1-mole charge transfer so the number of equivalents in one mole of sodium thiosulphate is 1.
The equivalent weight of sodium thiosulphate = molecular weight of sodium thiosulphate.
Molecular weight of sodium thiosulphate = $2\times \frac { 1 }{ 2 } \quad =\quad 1$
$2\times 23\quad +\quad 2\times 32\quad +\quad 3\times 16\quad +\quad 5\times 18\quad =\quad 248$

Therefore, the equivalent weight of sodium thiosulphate is 248. Option A is correct.

Note: We should take the number of electrons transferred by 2 sulphur atoms, not one sulphur atom because one mole of sodium thiosulphate contains 2 sulphur atoms. The equivalent mass calculation for acids and bases will be different.