A student investigates the relationship between molar mass and normal boiling point in the homologous series of straight-chain alkanes. The measured data are shown below.
n-Alkane
Chemical formula
Molar mass (g mol−1)
Normal boiling point (°C)
Propane
C3H8
44
–42
Butane
C4H10
58
–0.5
Pentane
C5H12
72
?
Hexane
C6H14
86
69
Respond to the following.
(a) Using the trend evident in the data, estimate the normal boiling point of n-pentane. Clearly describe your method.
(b) State the predominant type of intermolecular force responsible for the trend in boiling points across this series and briefly explain why the strength of that force changes with molar mass.
(c) 2-Methylbutane (iso-pentane) has the same molar mass as n-pentane. Predict whether its normal boiling point is higher than, lower than, or approximately equal to the value you reported in part (a). Justify your prediction in terms of molecular structure and intermolecular forces.