Oxides, Oxoanions, K_2Cr_2O_7 & KMnO_4

In acidic solution, potassium dichromate acts as a powerful oxidising agent, oxidizing sulphide ions ($\text{S}^{2-}$ or $\text{H}_2\text{S}$) to elemental colloidal sulphur ($\text{S}$):

The reduction half-reaction of permanganate ion to manganese(II) ion in acidic medium is:

From the stoichiometry:

• Reduction of $1\text{ mol}$ of $\text{MnO}_4^-$ to produce $1\text{ mol}$ of $\text{Mn}^{2+}$ requires $5\text{ moles}$ of electrons $= 5\text{ F}$ of charge.
• To obtain $2\text{ moles}$ of $\text{Mn}^{2+}$, the required charge is:
  $$Q = 2 \times 5\text{ F} = 10\text{ F}$$