Find the number of structural isomers possible for hexane ?

Which of the following hydrocarbons has stronger London dispersion forces ?

Identify the major product of following reaction.
$\text{Propene} \xrightarrow[\large{\text{Peroxide}}]{\large{\text{HBr}}} \,?$

Calculate the total volume of simple cubic unit cell in $\text{cm}^3$ if void volume is $2.0 \times 10^{-23}\,\text{cm}^3$