EBK EXPERIMENTAL ORGANIC CHEMISTRY: A M
EBK EXPERIMENTAL ORGANIC CHEMISTRY: A M
6th Edition
ISBN: 9781305687875
Author: Gilbert
Publisher: CENGAGE LEARNING - CONSIGNMENT
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Chapter 8.4, Problem 4E

(a)

Interpretation Introduction

Interpretation: While preparing a sample of UV-Vis analysis, if one assumes that the concentration of c, in methanol was 0.00600 g/L (True Value), against the actual value (Calculated Value) of 0.00576 g/L, the error in mg/L, in the mass of sample present in solutions needs to be determined.

Concept Introduction: The spectroscopy that uses light in the visible and adjacent range, giving absorption spectroscopy or reflectance spectroscopy, is termed as UV-Vis or Ultraviolet-Visible Spectroscopy.

(b) (i)

Interpretation Introduction

Interpretation: While preparing a sample of UV-Vis analysis, if one assumes that the concentration of c, in methanol was 0.00600 g/L (True Value), against the actual value (Calculated Value) of 0.00576 g/L, and assuming that a maximum was observed at a wavelength of 278 nm for this solution and that the value of A at this maximum was 0.623 when a 1-cm cell was used, and molar mass of 100 for ε , the value of ε needs to be calculated.

Concept Introduction: Depending on the chemical composition and structure of a chemical substance, light with a particular wave length is being absorbed by them, and a measure of such absorption of light, defining its strength is known by the term Molar Extinction Coefficient ( ε )

(b) (ii)

Interpretation Introduction

Interpretation: While preparing a sample of UV-Vis analysis, if one assumes that the concentration of c, in methanol was 0.00600 g/L (True Value), against the actual value (Calculated Value) of 0.00576 g/L, the information regarding the maximum value needs to be expressed.

Concept Introduction : When the value of molar extinction coefficient is known the value of log ε could be determined.

(b) (iii)

Interpretation Introduction

Interpretation: While preparing a sample of UV-Vis analysis, if one assumes that the concentration of c, in methanol was 0.00600 g/L (True Value), against the actual value (Calculated Value) of 0.00576 g/L, the % error in ε associated with the inaccuracy in assuming the actual concentration of the solution needs to be determined.

Concept Introduction: Molar concentration c can be derived by dividing the concentration of methanol by its molar mass

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B.  Quantitative Spectroscopy   Prepare two graphs, %T vs. Concentration and Absorbance vs. Concentration, using the data for the cobalt standards.  The concentrations of Cobalt standards 2-4 can be calculated using simple dilution calculations. Concentration should be on the x-axis for each of these graphs.  The first should clearly show some curvature while the second should be a linear curve. Using the absorbance graph you can determine the concentration of the unknown. Take the absorbance value for the unknown and find that value on the y-axis.  Draw a horizontal line from that point on the y-axis to the plotted line.  From the point where the horizontal line intersects the plotted line, draw a vertical line straight down to the x-axis.  Record the value from the point where you hit the x-axis as the unknown concentration.
True or False 1. Photodiode arrays are devices within the instrument that help amplify the signal of a sample. 2. Atomic spectroscopy requires the atomization of a sample, which involves nebulization and eventually volatilization to form an elemental vapor. 3. Organic compounds are normally analyzed in atomic spectroscopy despite their small electronic transition range.
Why must solutions with high concentrations be diluted prior to analysis via Beer's Law? A. The relationship between absorbance and concentration is not linear at high concentrations. B. The detector will reach its detection threshold. C. The photon source is too weak to provide accurate results. D. There is no need to work with dilute concentrations; any concentrations will work. E. The molar absorptivity of a compound is dependent on its concentration.
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