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.
- BThe detector will reach its detection threshhold.
- CThe molar absorptivity of a compound is dependent on its concentration.
- DThe relationship between absorbance and concentration is not linear at high concentrations.
- EThere is no need to work with dilute concentrations; any concentration will work.
Q2:
Which of the following would be the best solvent for a compound with a of 281 nm? You may assume that the compound is completely soluble in each.
- ABenzene (UV cutoff = 278 nm)
- BToluene (UV cutoff = 285 nm)
- CAcetone (UV cutoff = 329 nm)
- DEthanol (UV cutoff = 205 nm)
Q3:
-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 best practice?
- AOnly use two data points to ensure a straight line.
- BPrepare each sample “from scratch” to avoid cross contamination.
- CUse very concentrated solutions to ensure a good signal.
- DPrepare a stock solution and use 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?
- AUsing a cuvette with a shorter path length
- BDissolving more of the analyte in the solvent
- CPre-concentrating 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?
- ATriethylamine
- B 0.10 M triethylamine in water
- CWater
- DHexane
- E Air