Question

How long will it take for a uniform current of $6.00A$ to deposit $78g$ of gold from a solution of $AuC{l}_4^{-}$ ? What mass of chlorine gas will be formed simultaneously at anode of the cell?
(Atomic mass of $Au=197$)

• A. $t=3010sec$,
  $w=35.50g$
• B. $t=20306sec$,
  $w=45.54g$
• C. $t=19500sec$,
  $w=54.5g$
• D. $t=19139.16sec$,
  $w=42.24g$

Answer 1

Answer: (D)

Reactions take place at electrodes are :
At cathode : $AuC{l}_4^{-}+3e^{-}\to Au+4C{l}^{-}$
At anode : $Cl-\to \frac{1}{2}C{l}_2+e^{-}$
For the deposition of $197g$ ($1$ mole) of $Au=3F$ of charge is required thus, for the deposition of $78g$ of $Au$, charge required $=\frac{3}{197}\times 78=1.19F=1.19\times 96500$ coulombs
From $Q=I\times t$
$t=\frac{Q}{I}=\frac{1.19\times 96500}{6}$
$=19139.16sec$
By $1F$ charge, $35.5g$ of $C{l}_2$ gas is formed, thus from $1.19F$, mass of chlorine gas formed $=35.5\times 1.19=42.24g$