The correct pair of species which are not isostructural is

The rate of diffusion of methane at 1.0 atm pressure is twice than that of another gas ' $X$ ' kept at 1.45 atm . The molecular mass of the gas ' $X$ ' is

Which of the following gases has the maximum van der Waals' constant ' $a$ '?

Electrolysis of aqueous $\mathrm{Na}_2 \mathrm{SO}_4$ was carried out by passing a current of 3 ampere for 10 min . The volume of the gas (in litre) at STP at the anode of the cell is approximately