Vanillin,a compound obtained from vanilla beans,has a total sum of oxygen atoms and $\pi$-electrons equal to . . .

What is the molar mass of the third member of a homologous series if the molar mass of the first member is $46 \ g \ mol^{-1}$?

Fill in the blanks:
$(1)$ Two isomers of butane are $n$-butane and isobutane. The relation of the number of $\sigma$ and $\pi$-bonds in them is .......... .
$(2)$ .......... is present in the structure of a heterocyclic compound.
$(3)$ Cyclohexene is a .......... type and furan is a .......... type of compound.
$(4)$ .......... type of $3-D$ model indicates the atomic volume but does not indicate the bond.

Cyclohexa-$1,3$-diene contains . . . . . . vinylic and . . . . . . allylic hydrogen atoms?

What do the solid wedge and dashed wedge symbols indicate in the 3D representation of an organic molecule?

Write the importance of organic chemistry.