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# Mass Among The Homologs

APCHEM-TLLQOF

A series of compounds with the same general formula is called a homologous series. One such series is the alkenes. ${ { C }_{ 2 } }{ H }_{ 4 }$ (ethene), ${ C }_{ 3 }{ H }_{ 6 }$ (propene), and ${ C }_{ 4 }{ H }_{ 8 }$ (butene) are the first three members of the series.

The modified structural formulas for ethene and propene may be represented thus: $CH_2=CH_2$ and $CH_3CH=CH_2$.

There are two possible structures for butene, 1-butene and 2-butene, are named to indicate on which carbon atom the double bond begins. Note, however, that both have the same molecular formula: ${ C }_{ 4 }{ H }_{ 8 }$.

They are: $CH_2=CHCH_2CH_3$ and $CH_3CH=CHCH_3$.

As you can see based on the first three members of the series (the first three homologs), alkenes can be represented as a group by the general formula, ${ C }_{ n }{ H }_{ 2n }$, where $n$ is an integer and represents the number of carbon atoms in the molecule.

Suppose one were to graph the relationship between the molecular mass of the alkenes and the number of carbon atoms in the chain with the molecular mass on the vertical axis (dependent variable) and the number of carbon atoms (independent variable) on the horizontal axis.

Which of the following BEST illustrates the resulting plot?

A
B
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D