Arrange the following in increasing order of boiling point.
$(i)$ methane,butane,propane,pentane,ethane.
$(ii)$ $n$-pentane,neo-pentane,iso-pentane.
$(iii)$ ethane,$2$-methyl propane,propane,$n$-butane.
$(iv)$ $2$-methyl pentane,$2,2$-dimethyl butane,$2,2$-dimethyl propane.

(N/A) The boiling point of alkanes increases with an increase in molecular mass due to an increase in van der Waals forces. For isomers,the boiling point decreases with increased branching because the surface area decreases,leading to weaker intermolecular forces.
$(i)$ Methane $(CH_4)$ < Ethane $(C_2H_6)$ < Propane $(C_3H_8)$ < Butane $(C_4H_{10})$ < Pentane $(C_5H_{12})$.
$(ii)$ Neo-pentane (most branched) < Iso-pentane < $n$-pentane (straight chain).
$(iii)$ Ethane $(C_2H_6)$ < Propane $(C_3H_8)$ < $2$-methyl propane $(C_4H_{10})$ < $n$-butane $(C_4H_{10})$. Note: $n$-butane has a higher boiling point than its branched isomer $2$-methyl propane.
$(iv)$ $2,2$-dimethyl propane $(C_5H_{12})$ < $2,2$-dimethyl butane $(C_6H_{14})$ < $2$-methyl pentane $(C_6H_{14})$.