(a) CuCl2 is more stable than Cu2Cl2 because in CuCl2, Cu is in +2 oxidation state and Cu is in +1 oxidation state in Cu2Cl2. So, Cu+2 have a smaller size and thus high hydration enthalpy because of which it is more stable than Cu+.
(b) Atomic radii of 4d and 5d series elements are nearly same due to lanthanoid contraction. It is due to imperfect shielding of one of 4f electron by another of the same subshell. Consequently, the 2 series have almost similar size like Zr (160 pm) and Hf (159 pm).
(c) Hydrochloric acid is not used in permanganate titrations because HCl is much stronger acid than oxalic acid therefore, it reacts with KMnO4 to produce the chlorides of potassium and manganese liberating Chlorine gas. Due to this total amount of KMnO4 available for reaction decreases and hence, affects the redox titration.
KMnO4 + 16HCl ¾ 2KCl + 2MnCl2 + 8H2O + 5Cl2
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