The total number of 'Sigma' and 'Pi' bonds in 2-formylhex-4-enoic acid is _________.

The oxidation number of iron in the compound formed during brown ring test for NO$$_3^-$$ iron is ________.

Molality of 0.8 M H$$_2$$SO$$_4$$ solution (density 1.06 g cm$$^{-3}$$) is ________ $$\times10^{-3}$$ m.

Number of ways of arranging 8 identical books into 4 identical shelves where any number of shelves may remain empty is equal to