Worksheet: UV–Vis Sample Preparation

In this worksheet, we will practice describing UV–vis sample preparation, including filtration, dilution, cuvette selection, and reference selection.

Q1:

Why must solutions with high concentrations be diluted prior to analysis via Beer’s law?

  • AThe photon source is too weak to provide accurate results.
  • BThere is no need to work with dilute concentrations; any concentration will work.
  • CThe relationship between absorbance and concentration is not linear at high concentrations.
  • DThe detector will reach its detection threshold.
  • EThe molar absorptivity of a compound is dependent on its concentration.

Q2:

Which of the following would be the best solvent for a compound with a 𝜆max of 281 nm? You may assume that the compound is completely soluble in each.

  • AToluene (UV cutoff = 285 nm)
  • BBenzene (UV cutoff = 278 nm)
  • CAcetone (UV cutoff = 329 nm)
  • DEthanol (UV cutoff = 205 nm)

Q3:

𝛽 -carotene is a highly absorbing chromophore. Which solvent would be the best choice for analysis?

  • ABenzene
  • BMethylene bromide
  • CHexyne
  • DEthanol

Q4:

When preparing a calibration curve for sample analysis, which of the following is the best practice?

  • APreparing each sample “from scratch” to avoid cross contamination
  • BOnly using two data points to ensure a straight line
  • CUsing very concentrated solutions to ensure a good signal
  • DPreparing a stock solution and using serial dilution for the remaining standards

Q5:

When a sample is analyzed by UV–Vis spectroscopy, the results may be unreliable if the signal-to-noise ratio is too low. Which of the following is not a method for avoiding this issue?

  • ADissolving more of the analyte in the solvent
  • BUsing a cuvette with a shorter path length
  • CPreconcentrating the analyte solution
  • DUsing a solvent with a lower UV cutoff
  • EIncreasing the acquisition time for each absorbance measurement

Q6:

A sample of 0.10 M triethylamine in hexane is prepared for analysis. Which material should be used as the blank reference?

  • AHexane
  • BTriethylamine
  • CAir
  • DWater
  • E0.10 M triethylamine in water

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