Final answer:
In normal-phase HPLC, the least polar component of a mixture has the longest retention time, so in a mixture of an ester, a carboxylic acid, an alcohol, and an alkane, the alkane would have the longest retention time.
Step-by-step explanation:
In normal-phase high-performance liquid chromatography (HPLC), the stationary phase is typically polar, and the mobile phase is more non-polar. In this scenario, we have a mixture of an ester, a carboxylic acid, an alcohol, and an alkane. Among these compounds, the alkane is the least polar molecule. Since normal-phase HPLC separates based on polarity, more polar compounds adhere stronger and longer to the polar stationary phase and therefore will elute faster. Conversely, less polar compounds like alkanes interact weakly with the polar stationary phase and will typically have the longest retention time because they are more soluble in the non-polar mobile phase.
Therefore, in a mixture containing an ester, a carboxylic acid, an alcohol, and an alkane, separated by normal-phase HPLC, the alkane would have the longest retention time.