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Question Video: Identifying Which Property Compounds in the Same Homologous Series Have in Common Chemistry

Fill in the blank: A homologous series could be described as a family of compounds that have the same _.

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Video Transcript

Fill in the blank. A homologous series could be described as a family of compounds that have the same blank. (A) Physical properties, (B) molecular mass, (C) chemical properties, (D) number of carbon atoms, or (E) molecular formula.

A homologous series is a family of compounds that have the same functional group and general formula, with each compound in the series differing by a structural unit. Based on the definition, we could fill in the blank with functional group or a general formula. But neither of these phrases match the answer choices provided. So we’ll need to take a closer look at a homologous series.

Let’s take a look at the first three compounds in the alkane homologous series. Alkanes are molecules that only consist of single-bonded carbon and hydrogen atoms. And the members of the alkane homologous series differ by a CH2 structural unit. As each member of the alkane homologous series is one CH2 group larger than the previous molecule, the molecules of the series will not have the same number of carbon atoms, molecular formula, or molecular mass. So we can eliminate answer choices (B), (D), and (E).

The first three molecules in the alkane homologous series all exist as gases at room temperature. But as the alkanes get larger, they may exist as liquids or solids. So the physical state of molecules in a homologous series is not necessarily the same. In addition, physical properties such as boiling point are drastically different across the alkane homologous series. As the physical properties of molecules across a homologous series are not the same, we can eliminate answer choice (A).

This leaves us with answer choice (C) chemical properties. Members of the alkane homologous series are generally unreactive, but they do undergo combustion to produce carbon dioxide gas and water. As members of the alkane homologous series have similar reactivity, we can describe a homologous series as a family of compounds that have the same chemical properties. So we should fill in the blank with chemical properties, answer choice (C).

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