The equivalent mass of sodium thiosulphate (Na2S2O3·5H2O) in the reaction

2Na2S2O3 + I2 → 2NaI + Na2S4O6 is:

A) 248

B) 124

C) 596

D) 62

To determine the equivalent mass of sodium thiosulphate (Na2S2O3·5H2O) in the given reaction, we first need to understand the concept of equivalent mass and how it relates to the stoichiometry of the reaction.

Understanding Equivalent Mass

The equivalent mass of a substance is defined as the mass of that substance that will combine with or displace one mole of hydrogen atoms (or 8 grams of oxygen, or 35.5 grams of chlorine, etc.). In redox reactions, the equivalent mass can also be calculated based on the number of electrons transferred in the reaction.

Analyzing the Reaction

In the reaction provided:

• 2Na2S2O3 + I2 → 2NaI + Na2S4O6

We see that sodium thiosulphate (Na2S2O3) is acting as a reducing agent. Each mole of Na2S2O3 can donate electrons to iodine (I2), which is being reduced to iodide (NaI).

Calculating Molar Mass

First, let's calculate the molar mass of sodium thiosulphate pentahydrate (Na2S2O3·5H2O):

• Na: 22.99 g/mol × 2 = 45.98 g/mol
• S: 32.07 g/mol × 2 = 64.14 g/mol
• O: 16.00 g/mol × 3 = 48.00 g/mol
• H2O: (2 × 1.01) + 16.00 = 18.02 g/mol × 5 = 90.10 g/mol

Now, summing these values gives:

Molar mass of Na2S2O3·5H2O = 45.98 + 64.14 + 48.00 + 90.10 = 248.22 g/mol

Determining the Number of Equivalents

In this reaction, each mole of Na2S2O3 can donate 2 electrons (since it reduces I2 to NaI). Therefore, the number of equivalents of Na2S2O3 is equal to the number of moles multiplied by the number of electrons transferred:

Number of equivalents = moles × n

Here, n = 2 (the number of electrons transferred per mole of Na2S2O3).

Calculating Equivalent Mass

The equivalent mass can be calculated using the formula:

Equivalent mass = Molar mass / n

Substituting the values we have:

Equivalent mass = 248.22 g/mol / 2 = 124.11 g/equiv

Final Answer

Thus, the equivalent mass of sodium thiosulphate (Na2S2O3·5H2O) in the reaction is approximately 124 g/equiv. Therefore, the correct answer is B) 124.