In this example, you'll learn how hydrogen bonding affects boiling point. The problem reads, one of the compounds shown here is a liquid at room temperature. Which one and why? First, determine the molar mass of each compound.
The molar masses are shown here. Molar mass correlates, roughly, with the strength of the dispersion force. Since the three compounds have similar molar masses, the strengths of their dispersion forces are similar.
All three compounds are also polar. so they all have dipole-dipole forces. Recall that polar molecules containing hydrogen atoms bonded directly to small electronegative atoms, most importantly, fluorine, oxygen, or nitrogen, exhibit hydrogen bonding.
Of the three molecules in this example, only hydrogen peroxide includes hydrogen bonded directly to fluorine, oxygen, or nitrogen. Therefore, it also has hydrogen bonding and is likely to have the highest boiling point of the three. Since the example stated that only one of the compounds is a liquid, you can safely assume that hydrogen peroxide is the liquid. Formaldehyde also contains hydrogen and oxygen. Why does it not have hydrogen bonding?
A. Because it contains carbon. B.
Because the hydrogen atom is not directly bonded to the oxygen atom. Or C. Because it contains a double bond. The correct answer is B. Formaldehyde does not have hydrogen bonding because the hydrogen atom is not directly bonded to the oxygen atom. Although formaldehyde contains both hydrogen and oxygen, hydrogen is not directly bonded to oxygen.
So formaldehyde does not have hydrogen bonding either. Notice that although fluoromethane contains both hydrogen and fluorine, hydrogen is not directly bonded to fluorine. So fluoromethane does not have hydrogen bonding as an intermolecular force either.