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If the have similar shape and molar mass but different boiling points, they must have different intermolecular forces

Chemistry

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If the have similar shape and molar mass but different boiling points, they must have different intermolecular forces.

Consider the compounds propane, dimethyl ether and ethanol.

Propane, ##"CH"_3"CH"_2"CH"_3##: molar mass = 44 g/mol; boiling point = -42 °C

Dimethyl ether, ##"CH"_3"OCH"_3##: molar mass = 46 g/mol; boiling point = -24 °C

Ethanol, ##"CH"_3"CH"_2"OH"##: molar mass = 46 g/mol; boiling point = 78 °C

These compounds all consist of a chain of three "heavy" atoms with hydrogen atoms attached, and they have similar molar masses, but widely different boiling points.

Propane is a nonpolar compound. It has only weak London dispersion forces, so the molecules can easily escape into the gas phase.

It has a low boiling point and is a gas at room temperature.

Dimethyl ether has polar ##"C-O"## bonds, so its intermolecular forces are the stronger dipole-dipole attractions.

It has a higher boiling point, but it is still a gas at room temperature.

Ethanol has strong . The intermolecular forces are so strong that ethanol is a liquid at room temperature.