Question

You know that 1 F is charge of 1 mole of electrons. How many moles of a trivalent cation would 5 F reduce?

• A. $3$
• B. $\frac{5}{3}$
• C. $15$
• D. $\frac{3}{5}$

Answer 1

The correct option is B $\frac{5}{3}$

To find the number of moles of cation which is getting reduced or anion which is getting oxidized, always write down the half-reaction for practice.
For example, for Aluminium, $Al\to A{l}^{3+}+3e^{-}$

To find for 5 F, I'll try to make the electrons 5 by multiplying by 5 and dividing by 2 on both sides.
So, $\frac{5}{3}Al\to \frac{5}{3}A{l}^{3+}+5e^{-}$

Therefore, we get $\frac{5}{3}$ moles of Al for 5 moles of electrons which is 5 Faradays.

This is exactly the concept of equivalents. Simply put, it is the amount of substance which reacts with 1 F in electrochemistry terms.