## Mole Concept Redox Questions

Here’s an “ingelious” way to solve redox questions related to mole concept.

Let’s look at a typical JC A Level H2 Chemistry question:

50 cm^{3} of a 0.10 moldm^{-3} solution of a metallic salt was found to react exactly with 25.0 cm^{3} of a 0.10 moldm^{-3} aqueous sodium sulfite. In this reaction, the sulfite ion is oxidised as follows:

SO_{3}^{2-} (aq) + H_{2}O(l) SO_{4}^{2-} (aq) + 2H^{+} + 2e^{–}

What is the new oxidation number of the metal in the salt if its original oxidation number was +3?

The trick is to first ignore if the metal gets oxidised or reduced.

First decide how many moles of electrons gain/lose for each metallic ion.

Amount of sulfite ions = x 25 = 0.0025 mol

Amount of metallic salt = x 50 = 0.0050 mol

Find the total number of electrons transferred = 0.0025 mol x 2 = 0.005 mol

Find the total number of electrons gain/lose for each metallic ion = = 1

Then decide if the metal gets oxidised or reduced.

Since sulfite was oxidised, the metallic ion gets reduced (by 1).

As the original oxidation number of the metal was +3, the new oxidation number is now +2.

As an analogy, consider there are 25 boys in the class.

Each boy will pass \$2 to the girls in the class for a total of \$50 dollars.

If there are 50 girls in the class, each girl will receive \$1.

Since each girl has \$2 from the start, they will now have \$3 each.