Passage

Passage
A group of students were given water from two reservoirs and asked to determine water hardness using flame atomic absorbance (AA) spectroscopy.  Water hardness can be found by relating the concentration of calcium and magnesium ions to the concentration of carbonate species in the sample using Equation 1.[CO₃^2-] = 2.5[Ca²⁺] + 4.1[Mg²⁺]Equation 1To correlate the concentration of the metal ions to absorbance, the students made standard solutions of MgO (MW = 40.31 g/mol) and CaO (MW = 56.08 g/mol) by diluting small portions of 0.01 M MgO and 0.01 M CaO stock solutions with 2% HNO₃ into 100 mL volumetric flasks.  The calibration curves generated from the standard solutions are shown in Figures 1 and 2.
Figure 1  Calibration curve from the absorption of the Ca²⁺ solution standards
Figure 2  Calibration curve from the absorption of the Mg²⁺ solution standardsLanthanum(III) chloride (MM = 245.25) can be used to eliminate chemical interference in a sample by competitively binding to PO₄³⁻, releasing calcium to be atomized in the flame for detection.The students prepared unknown Samples 1 and 2 by mixing 25 mL reservoir water, 16 mL of 2% HNO₃, and 9 mL of 0.1 M LaCl₃ for a total sample volume of 50 mL each.  The samples were each tested three times using flame AA spectroscopy, yielding the results reported in Table 1.Table 1  AA Spectroscopy Results for Reservoir Water Samples

-The passage states that the standard solutions were prepared for analysis by diluting small portions of the stock solutions.  How does the concentration of the MgO standard solution with the lowest nonzero absorbance compare to the concentration of the MgO stock solution?

A) It is smaller by a factor of 1 × 10⁻⁶.
B) It is smaller by a factor of 1 × 10¹.
C) It is smaller by a factor of 1 × 10².
D) It is smaller by a factor of 1 × 10³.

Correct Answer:

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