Q.vii. What is the oxidation state of S in H$_2$SO$_4$.

SO3 is trigonal planar with three sigma bonds and no lone pair on S -> sp2 hybridisation.

In sulphur trioxide, the central sulphur atom forms three sigma bonds to three oxygen atoms and has no lone pair. The number of electron domains around S is 3, which corresponds to sp2 hybridisation and a trigonal …

Oleum, or fuming sulphuric acid, has the formula $H_2S_2O_7$.

When sulphur trioxide ($SO_3$) is absorbed in concentrated sulphuric acid ($H_2SO_4$) during the Contact Process, it forms pyrosulphuric acid, commonly called oleum:

$H_2SO_4 + SO_3 \longrightarrow H_2S_2O_7 \ (\text{oleum})$

Oleum is a fuming liquid because on contact with atmospheric moisture it regenerates $H_2SO_4$, releasing visible white fumes:

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H2SO4 gives two H+ ions, so it is dibasic.

Basicity of an acid = number of replaceable hydrogen atoms (ionisable protons) it can furnish. H2SO4 ionises in two steps:

H2SO4 → H+ + HSO4^-

HSO4^- → H+ + SO4^2-

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Oleum = H2SO4 + SO3 (fuming sulphuric acid, H2S2O7).

In the Contact process, SO3 is absorbed in concentrated H2SO4 to give oleum (pyrosulphuric acid, H2S2O7), rather than being absorbed directly in water:

H2SO4 + SO3 → H2S2O7 (oleum)

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