Rank the following compounds in order of increasing strength of intermolecular forces:
a. ch3ch2ch2cho (butanal)
b. ch3ch2ch2ch2oh (1−butanol)
c. ch3ch2ch2ch2ch3 (pentane)

pentane < butanal < 1-butanol

Explanation:

The intermolecular forces are the forces that bind together the molecules in a substance, and its strength depends on the polarity of the molecule. The polarity is given by the dipole moment, which is the vector sum of the dipoles of the bonds. The dipole is the difference of electronegativity between the elements of the bond. So, if the dipole moment is 0, the molecule is nonpolar, and if it's different from 0 the molecule is polar and has partial charges.

The nonpolar molecules have the weakest force, which is called London forces, or dipole induced-dipole induced forces. The polar molecules have dipole-dipole forces, which is stronger than the London forces. If the polar molecules have the hydrogen-bonded to a high electronegative element, thus, this dipole-dipole bond is extremely strong, and it's called a hydrogen bond.

Butanal has the oxygen bonded to carbon, and the hydrogen-bonded to carbon too. The dipole at carbon-carbon is 0, and the dipole of H-C is canceled, but the dipole of C=O is different from 0, so, the molecule is polar, and has dipole-dipole force.

1-butanol has a bond O-H, so it has a hydrogen bond, and pentane only has bonds between C-C, which are nonpolar, and the bonds C-H have its dipole canceled, thus the molecule is nonpolar, and has London force. So, the order of increasing strength is:

pentane < butanal < 1-butanol