Question

Two compounds have the same empirical formula. One substance is a gas, the other is a viscous liquid. How is it possible for two substances with the same empirical formula to have markedly different properties?

Short answer

Two substances with the same empirical formula can have markedly different properties due to differences in their molecular structure and the type of chemical bonds within the molecules. Molecular structure affects a compound's physical and chemical properties, and the state of matter (gas or liquid) is influenced by intermolecular forces that depend on the molecular structure. In this case, the gas substance likely forms simple non-polar molecules with weak intermolecular forces, while the viscous liquid might have more complex, polar molecules with strong intermolecular forces, leading to their distinct properties.

Step-by-step solution

Define an empirical formula

An empirical formula represents the simplest ratio of elements in a compound (e.g., CH for methane and ethyne). It doesn't provide any information about the actual number of atoms or the molecular structure of the compound.

Explain molecular structure

Molecular structure refers to the specific arrangement of atoms in a molecule and the type of chemical bonds connecting those atoms. Even if two substances share the same empirical formula, their molecular structures can be different. For example, isomers are molecules with the same molecular formula but different molecular structures. This difference in molecular structure can lead to markedly different physical and chemical properties.

Differentiate between gas and liquid

The state of matter, such as gas or liquid, is affected by molecular structure and intermolecular forces. Gas molecules are widely separated and exhibit weak intermolecular forces, while liquid molecules exhibit stronger intermolecular forces that hold the molecules closer together.

Relate molecular structure to visible properties

Given that one substance is a gas and the other is a viscous liquid, the molecular structure and type of chemical bonds between the atoms within each molecule and the intermolecular forces between the molecules must be drastically different. This results in the two substances having greatly different properties despite their identical empirical formulas. A possible reason could be that the gas substance forms simple non-polar molecules that have weak intermolecular forces, while the viscous liquid might have more complex, polar molecules with strong intermolecular forces.