19. Although Zr belongs to 4d and Hf belongs to the 5d transition series but it is quite difficult to separate them. Why? 

Electronic configuration of

Gd is ; Gd₆₄ = [Xe]4f⁷ 5d¹ 6s²

Hence $G{d}_{64}^{++}=\left[Xe\right]4f^{7}5d^{1}6s^0$

Total number of electrons in 4f sub-shell = 7

Ans. = 7

Molecular formula of Mohr’ salt, FeSO₄. (NH₄)₂SO₄.6H₂O.

Molecular formula of potash alum = K₂SO₄.Al₂ (SO₄)₃.24H₂O. Overall molecular formula of potash alum = KAl (SO₄)₂. 12H₂O. Ratio of water molecules in Mohr’ salt and Potash alum

$=\frac{6}{12}=\frac{1}{2}=0.5=5\times 10^{-1}$       

Ans. = 5

Cu₂Cl₂ + Water -> Insoluble

AgCl + Water -> Insoluble

ZnCl₂ + Water -> Colouless solution

CuCl₂ + Water -> Coloured solution due to 3d⁹ electronic configuration.

Nucleophilicity is directly proportional to the electron density of atom and inversely proportional to its electronegativity.

According to Ellingham diagram, the metal oxide with lower $\Delta G^o$ is more stable than metal oxide of higher $\Delta G^o$ at same temperature.