Passage II

Biodiesel (BD) is a renewable alternative to tradi­tional petroleum diesel (PD). BD is typically prepared by reacting soybean oil with methanol in the presence of a cat­alyst, forming compounds called fatty acid methyl esters(FAMEs). In contrast, PD contains no FAMEs. The pres­ence of FAMEs in BD causes BD to absorb infrared light differently than does PD. This difference allows pure BD, pure PD, and mixtures of BD and PD to be distinguished by analyzing the absorbance of infrared light.

Students performed 3 studies in which they deter­mined the infrared absorbance characteristics of pure BD, pure PD, and mixtures of BD and PD.

Study 1

The students measured the absorbance, A, of a sample of pure BD and a sample of pure PD at wavenumbers from 600 cm-1 through 1,800 cm-1. The wavenumber corresponding to a given wavelength is defined as where the wavelength is given in cm and the resulting wavenumber is given in cm-1. They plotted the results for each sample (see Figure 1).

Study 2

The students prepared 7 different mixtures of BD and PD, each containing a different percent by volume of BD. Then, they measured A at 1,746 cm-1 for a sample of each of the 7 mixtures, a sample of pure BD, and a sample of pure PD (see Figure 2).

Figures 1 and 2 adapted from A. P. Ault and R. Pomery, "Quantita­tive Investigations of Biodiesel Fuel Using Infrared Spectroscopy: An Instrumental Analysis Experiment for Undergraduate Chemistry Students."©2011 by Division of Chemical Education, Inc., Ameri­can Chemical Society.

Study 3

The students obtained 4 different samples of commer­cial fuel blends of BD and PD (Samples W-Z). They mea­sured A at 1,746 cm-1 for each sample, and then used Figure 2 to calculate the percent BD by volume of each sample (see Table 1).

Table 1 adapted from Z. V. Feng and J. T. Buchman, "Instrumental Analysis of Biodiesel Content in Commercial Diesel Blends: An Experiment for Undergraduate Analytical Chemistry."©2012 by Division of Chemical Education, Inc., American Chemical Society.

11. Suppose that in Study 1 the students had measured the absorbance at wavenumbers from 600 cm-1 through only 1,600 cm-1 (instead of through 1,800 cm-1). Based on Figure 1, would the students more likely have measured the absorbance in Study 2 at a wavenumber of 1,172 cm-1or at a wavenumber of 1,464 cm-1?

Answer and Explanation

Your Answer is

Correct Answer is B

Explanation

First of all, it is necessary to understand why the absorbance at the wavelength of 1746 cm-1 is measured in figure 2. It can be seen from figure 1 that at the wavelength of 1746 cm-1, the absorbance of BD and PD is significantly different, so it is easiest to distinguish these two substances;

Looking at figure 1, it is found that when the abscissa When the wavenumber is in the range of 1400-1500, the absorbance of BD and PD is very close, so it is difficult to distinguish BD and PD. Therefore, it is recommended to measure the wavelength of 1172 cm-1, because at this time the distinction between BD and PD is obvious, and the absorbance of BD is much higher than that of PD, so it is easy to detect the influence of the change of BD content in the mixture on the absorbance