Q.(i) Calculate effective atomic number of Fe$^{2\oplus}$ in [Fe(CN)$_6$]$^{4\ominus}$ [Given: (Z = 26)].

Fe$^{2+}$ attains an EAN of 36 (krypton configuration) in the hexacyanoferrate(II) ion; the lead accumulator is a Pb/PbO$_2$/H$_2$SO$_4$ cell; H$_2$–O$_2$ fuel cells are clean and continuous power sources.

(i) EAN of Fe$^{2+}$ in $[Fe(CN)_6]^{4-}$: Fe has $Z = 26$, so $Fe^{2+}$ has $26 - 2 = 24$ electrons. Each of the 6 $CN^-$ ligands is a 2-electron donor, contributing $6 \times 2 = 12$ electrons via coordinate bonds.

$EAN = 24 + 12 = 36$

(This equals the atomic number of krypton, the next noble gas — consistent with the effective atomic number rule.)

(ii) Lead accumulator (lead–acid battery): It consists of a lead (Pb) plate as the anode (negative electrode) and a lead dioxide ($PbO_2$) plate as the cathode (positive electrode), with alternating sets of plates immersed in aqueous sulfuric acid ($\sim$38% $H_2SO_4$) as electrolyte, held in a container/case; the plates are connected externally to deliver current. During discharge, both electrodes convert to lead sulfate ($PbSO_4$), and the process is reversed on recharging.

(iii) Two advantages of the $H_2-O_2$ fuel cell:

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