A mercury button cell works by $Zn (Hg) + HgO ZnO + Hg$ with zinc in excess. The cell contains $2.17 g$ of $HgO$ . Find the theoretical capacity of the cell in $mAh$ (nearest integer). Take Hg = 201, O = 16, $F = 96500 C mo l^{- 1}$ , $1 mAh = 3.6 C$ .

Numerical answer type, answer in mAh, tolerance 2.

Answer: 536 mAh

Cathode:

HgO + H X_{2} O + 2 e X^{-} Hg + 2 OH X^{-}

, so each mole of

HgO

accepts

2 mol

of electrons. Moles of

HgO = 2.17/217 = 0.0100 mol

, charge

= 2 \times 0.0100 \times 96500 = 1930 C

. Capacity

= 1930/3.6 = 536 mAh

. Since zinc is in excess,

HgO

limits the capacity.