The electronic configurations of bivalent europium and trivalent cerium are
(atomic number : Xe = 54, Ce = 58, Eu = 63)

The hardness of a water sample containing 10^–3 M MgSO₄ expressed as CaCO₃ equivalents (in ppm) is ______.
(molar mass of MgSO₄ is 120.37 g/mol)

The molarity of HNO₃ in a sample which has density 1.4 g/mL and mass percentage of 63% is _____.
(Molecular Weight of HNO₃ = 63)

108 g of silver (molar mass 108 g mol^–1) is deposited at cathode from AgNO₃(aq) solution by a certain quantity of electricity. The volume (in L) of oxygen gas produced at 273 K and 1 bar pressure from water by the same quantity of electricity is _______.