AP Chemistry
3.9 Separation of Solutions and Mixtures
3.1 Intermolecular and Interparticle Forces
A student uses thin-layer chromatography to compare three neutral analytes, \(P\), \(Q\), and \(R\), that are similar in size and differ primarily in molecular polarity. Separate plates coated with a polar silica-gel stationary phase are developed using the mobile phases shown in the table. Development is stopped at different times, so the solvent-front distances differ.
The retention factor is defined as
\[ R_f=\dfrac{\text{distance traveled by analyte}}{\text{distance traveled by solvent front}} \]
Which choice best identifies the most polar analyte and explains the effect of replacing hexane with ethyl acetate?
| Mobile phase | Dielectric constant, \(\varepsilon_r\) | Solvent-front distance \((\text{cm})\) | Distance traveled by \(P\) \((\text{cm})\) | Distance traveled by \(Q\) \((\text{cm})\) | Distance traveled by \(R\) \((\text{cm})\) |
|---|---|---|---|---|---|
| Hexane, \(\text{C}_6\text{H}_{14}\) | \(1.9\) | \(8.0\) | \(6.4\) | \(3.2\) | \(1.6\) |
| Ethyl acetate, \(\text{CH}_3\text{COOCH}_2\text{CH}_3\) | \(6.0\) | \(5.0\) | \(4.5\) | \(3.5\) | \(2.0\) |
The retention factor is defined as
\[ R_f=\dfrac{\text{distance traveled by analyte}}{\text{distance traveled by solvent front}} \]
Which choice best identifies the most polar analyte and explains the effect of replacing hexane with ethyl acetate?
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