Three faradays of electricity were passed through an aqueous solution of iron(II) bromide. What mass of iron (atomic mass 56) is deposited at the cathode?
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → 84 g
By Faraday’s first law, the amount deposited is proportional to the charge passed; one faraday is the charge on one mole of electrons. At the cathode, iron(II) ions take up two electrons each:
\( \displaystyle \ce{Fe^2+(aq) + 2e^- -> Fe(s)} \)
Electrons needed: 2 mol of electrons (2 F) deposit 1 mol (56 g) of iron.
Moles of iron from 3 F:
\( \displaystyle n_{\ce{Fe}}=\frac{3\,\mathrm{F}}{2\,\mathrm{F\,mol^{-1}}}=1.5\,\mathrm{mol} \)
Mass of iron:
\( \displaystyle m=1.5\,\mathrm{mol}\times56\,\mathrm{g\,mol^{-1}}=84\,\mathrm{g} \)