`100mL` of `0.3 M Fe^(3+)(aq)` ions were electrolyzed by a charge of `0.072F.` In electrolysis, metal was deposited and `O_(2)(g)` was evolved. At the end of electrolysis, it is desired to oxidize the un`-`electrolyzed metal ion.
`Fe^(3+)+e^(-) rarrFe^(2+)`
`Fe^(2+)+2e^(-)rarrFe`
The moles of `Fe^(2+)` ions left un`-`electrolyzed in the solution is
`a. 0.009" "b.0.021" "c.0.072" "d.0.042`

Explanation

To solve the problem, we need to calculate the moles of un-electrolyzed Fe²⁺ ions left in the solution after the electrolysis of Fe³⁺ ions. Let's break down the solution step by step.

### Step 1: Calculate the initial moles of Fe³⁺ ions
Given:
- Volume of solution = 100 mL = 0.1 L
- Molarity of Fe³⁺ = 0.3 M

Using the formula for moles:
...