Question

a student placed a sample of a food coloring that contains a mixture of a blue dye and a red dye at the top of a chromatography column filled with a nonpolar stationary phase. when water is poured through the column, two bands of colors are seen in the column, as shown in the diagram above.

1. which of the following will most likely be observed if ( mathrm{ch}_{3} mathrm{oh}(l) ) is used in the column instead of water? (assume that both solvents flow through the column at the same rate.)

(a) the dyes will not pass through the column because ( mathrm{ch}_{3} mathrm{oh}(l) ) is a nonpolar solvent.
(b) the dyes will pass through the column at a slower rate because ( mathrm{ch}_{3} mathrm{oh}(l) ) is a less polar solvent than water.
(c) the dyes will pass through the column at a faster rate because ( mathrm{ch}_{3} mathrm{oh}(l) ) has a greater molar mass than water.
(d) no differences will be observed because the polarity of the solvent does not affect the separation of the dyes.

Explanation:

Step1: Analyze the polarity of solvents

Water is a highly polar solvent. \(CH_3OH(l)\) (methanol) is less polar than water. In chromatography, the interaction between the solvent (mobile phase) and the solute (dyes) affects the movement of solute through the column. A less polar solvent has weaker interactions with polar solutes.

Step2: Consider the effect on dye movement

Since the stationary phase is non - polar, when a less polar solvent (\(CH_3OH(l)\)) is used instead of water (more polar), the dyes (which have some polarity) will have less interaction with the less polar solvent. But the key is that the less polar solvent \(CH_3OH(l)\) has a weaker ability to compete with the non - polar stationary phase for the dyes compared to water. However, the main factor for the rate of passing through the column: the interaction between the mobile phase (solvent) and the solute (dyes). A less polar solvent (\(CH_3OH(l)\)) has a weaker interaction with polar dyes (assuming the dyes have some polarity) than water. But in terms of the balance between interaction with stationary phase (non - polar) and mobile phase. The less polar \(CH_3OH(l)\) will not hold the dyes as strongly as water (because water, being more polar, has a stronger interaction with polar parts of the dyes if they are polar). But actually, the rate at which the dyes pass through is related to the solubility in the mobile phase and adsorption on the stationary phase. A less polar mobile phase (\(CH_3OH(l)\)) will make the dyes (if they have some polarity) have less solubility in the mobile phase (compared to water - mobile phase) but also less adsorption on the non - polar stationary phase. However, the dominant factor is that \(CH_3OH(l)\) is less polar than water. The dyes (assuming they have some polarity) have a stronger interaction with water (more polar mobile phase) than with \(CH_3OH(l)\). So, when using \(CH_3OH(l)\) (less polar mobile phase), the dyes will have a weaker interaction with the mobile phase and a relatively stronger interaction with the non - polar stationary phase. But wait, no. Wait, in chromatography, the more similar the polarity of the mobile phase and the solute, the faster the solute moves. Since \(CH_3OH(l)\) is less polar than water, and if the dyes have some polarity (but not as polar as water - solute interaction). The interaction between \(CH_3OH(l)\) (mobile phase) and dyes is weaker than water - dyes interaction. But the stationary phase is non - polar. The dyes will have a balance between being in the mobile phase and adsorbed on the stationary phase. A less polar mobile phase (\(CH_3OH(l)\)) means that the dyes (if they have some polarity) are less likely to be in the mobile phase (compared to when water is the mobile phase). But actually, the correct reasoning is: In chromatography, the elution power of a solvent depends on its polarity. A less polar solvent (\(CH_3OH(l)\) is less polar than water) will elute the solutes (dyes) more slowly because the solutes (assuming they have some polarity, and water, being more polar, can 'pull' the polar parts of the dyes into the mobile phase better). So, when \(CH_3OH(l)\) (less polar) is used as the mobile phase, the dyes (which interact more with a polar mobile phase) will pass through the column more slowly.

Answer:

B. The dyes will pass through the column at a slower rate because \(CH_3OH(l)\) is a less polar solvent than water.