Which of the following compounds contain(s) no covalent bond(s)? KCl, PH₃, O₂, B₂H₆, H₂SO₄

The combustion of benzene(l) gives CO₂(g) and H₂O(l). Given that heat of combustion of benzene at constant volume is –3263.9 kJ mol^–1 at 25^oC; heat of combustion (in kJ mol^–1) of benzene at constant pressure will be :
(R = 8.314 JK^–1 mol^–1)

How long (approximate) should water be electrolysed by passing through 100 amperes current so that the oxygen released can completely burn 27.66 g of diborane?
(Atomic weight of B = 10.8 u)

An aqueous solution contains 0.10 M H₂S and 0.20 M HCl. If the equilibrium constants for the formation of HS^– from H₂S is 1.0 $$\times$$ 10^–7 and that of S^2- from HS^– ions is 1.2 $$\times$$ 10^–13 then the concentration of S^2- ions in aqueous solution is :