unit 3 college board questions
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.
45. 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.
properties of ( mathrm{ch}_{3} mathrm{oh} ) and ( mathrm{h}_{2} mathrm{o} ) are given in the table. a student is given a beaker containing a solution of ( mathrm{ch}_{3} mathrm{oh}(l) ) and ( mathrm{h}_{2} mathrm{o}(l) )

46. if the student wishes to separate and collect the two components of the solution, which of the following experimental techniques would be most appropriate?
(a) using paper chromatography and collecting the liquid that first moves to the top of the paper
(b) combusting the ( mathrm{ch}_{3} mathrm{oh} ) and collecting the products of the reaction
(c) boiling the mixture and condensing and collecting the first vapors produced
(d) titrating the mixture with ( mathrm{hcl}(a q) ) to the equivalence point

Question

unit 3 college board questions
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.
45. 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.
properties of ( mathrm{ch}_{3} mathrm{oh} ) and ( mathrm{h}_{2} mathrm{o} ) are given in the table. a student is given a beaker containing a solution of ( mathrm{ch}_{3} mathrm{oh}(l) ) and ( mathrm{h}_{2} mathrm{o}(l) )

46. if the student wishes to separate and collect the two components of the solution, which of the following experimental techniques would be most appropriate?
(a) using paper chromatography and collecting the liquid that first moves to the top of the paper
(b) combusting the ( mathrm{ch}_{3} mathrm{oh} ) and collecting the products of the reaction
(c) boiling the mixture and condensing and collecting the first vapors produced
(d) titrating the mixture with ( mathrm{hcl}(a q) ) to the equivalence point

Accepted Answer

# Brief Explanations:
- **Question 45**:
    - Chromatography separation is based on the interaction between the solute (dyes), the stationary phase (non - polar in this case), and the mobile phase (solvent).
    - Water ($H_2O$) is a polar solvent. Methanol ($CH_3OH$) is less polar than water (as can be inferred from its lower $K_a$ value which is related to its ability to donate protons in a polar - solvent - like behavior and also from its structure with a non - polar $ - CH_3$ group).
    - A less polar solvent will have a weaker interaction with the polar dyes (assuming the dyes have some polar character). Since the stationary phase is non - polar, the attraction of the dyes to the stationary phase will be relatively stronger compared to their attraction to the less - polar mobile phase ($CH_3OH$) than to water. So, the dyes will pass through the column at a slower rate.
    - Option A is wrong because $CH_3OH$ is a polar (though less polar than water) solvent, not non - polar. Option C is wrong because molar mass does not directly determine the rate of movement in chromatography based on the principles of separation (polarity and interaction with stationary/mobile phases are key). Option D is wrong because polarity of the solvent does affect the separation as it influences the solute - solvent and solute - stationary - phase interactions.
 - **Question 46**:
    - **Option A**: Paper chromatography is used for separating components based on their different affinities for the stationary (paper) and mobile phases. But it is not used for collecting the components in a pure form (it is more for separation and identification).
    - **Option B**: Combusting $CH_3OH$ will change the chemical nature of $CH_3OH$ (it will react with $O_2$ to form $CO_2$ and $H_2O$), so we cannot collect the original components.
    - **Option C**: Distillation (boiling the mixture and condensing and collecting the first vapors) is a valid method. Since $CH_3OH$ has a lower boiling point ($65^{\circ}C$) than $H_2O$ ($100^{\circ}C$), when we boil the mixture, $CH_3OH$ will vaporize first. We can condense and collect these vapors to get relatively pure $CH_3OH$, and then collect $H_2O$ after.
    - **Option D**: Titrating with $HCl(aq)$ is used for acid - base reactions (to determine the concentration of an acid or base). It is not a separation technique for $CH_3OH$ and $H_2O$.

# Answer:
45. B. The dyes will pass through the column at a slower rate because $CH_3OH(l)$ is a less polar solvent than water.
46. C. Boiling the mixture and condensing and collecting the first vapors produced.

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