To determine the quantity of charge, in Faraday units, required for the reduction of 3.5 moles of Cr2O72− in acid medium, we need to look at the half-reaction for the reduction of Cr2O72− to Cr3+ : Cr2O72−+14H++6e−⟶2Cr3++7H2O The balanced equation shows that 6 moles of electrons (e−)are needed to reduce 1 mole of Cr2O72−. Therefore, for 3.5 moles of Cr2O72− : The total charge required in moles of electrons is calculated as: Total charge =3.5 moles \ of Cr2O72−×6 moles of e−∕mole \Cr2O72− So: Total charge =3.5×6=21 Faradays Thus, the quantity of charge required for the reduction of 3.5 moles of Cr2O72− in acid medium is 21.0 Faradays. The correct answer is Option C: 21.0 .