Multiple Choice
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.[CO32-] = 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.
![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.[CO<sub>3</sub><sup>2-</sup>] = 2.5[Ca<sup>2+</sup>] + 4.1[Mg<sup>2+</sup>]<strong>Equation 1</strong>To 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% HNO<sub>3</sub> into 100 mL volumetric flasks. The calibration curves generated from the standard solutions are shown in Figures 1 and 2. <strong>Figure 1</strong> Calibration curve from the absorption of the Ca<sup>2+</sup> solution standards <strong>Figure 2</strong> Calibration curve from the absorption of the Mg<sup>2+</sup> solution standardsLanthanum(III) chloride (MM = 245.25) can be used to eliminate chemical interference in a sample by competitively binding to PO<sub>4</sub><sup>3−</sup>, 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% HNO<sub>3</sub>, and 9 mL of 0.1 M LaCl<sub>3</sub> 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.<strong>Table 1</strong> AA Spectroscopy Results for Reservoir Water Samples -How many La<sup>3+</sup> ions are present in each of the unknown samples? A) 1.5 × 10<sup>−27</sup> B) 5.4 × 10<sup>20</sup> C) 9.0 × 10<sup>20</sup> D) 6.0 × 10<sup>22</sup>](https://d2lvgg3v3hfg70.cloudfront.net/MD0008/11ecba2d_11cf_8576_a3bf_6bc7e9116f7e_MD0008_00.jpg)
Figure 1 Calibration curve from the absorption of the Ca2+ solution standards
![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.[CO<sub>3</sub><sup>2-</sup>] = 2.5[Ca<sup>2+</sup>] + 4.1[Mg<sup>2+</sup>]<strong>Equation 1</strong>To 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% HNO<sub>3</sub> into 100 mL volumetric flasks. The calibration curves generated from the standard solutions are shown in Figures 1 and 2. <strong>Figure 1</strong> Calibration curve from the absorption of the Ca<sup>2+</sup> solution standards <strong>Figure 2</strong> Calibration curve from the absorption of the Mg<sup>2+</sup> solution standardsLanthanum(III) chloride (MM = 245.25) can be used to eliminate chemical interference in a sample by competitively binding to PO<sub>4</sub><sup>3−</sup>, 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% HNO<sub>3</sub>, and 9 mL of 0.1 M LaCl<sub>3</sub> 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.<strong>Table 1</strong> AA Spectroscopy Results for Reservoir Water Samples -How many La<sup>3+</sup> ions are present in each of the unknown samples? A) 1.5 × 10<sup>−27</sup> B) 5.4 × 10<sup>20</sup> C) 9.0 × 10<sup>20</sup> D) 6.0 × 10<sup>22</sup>](https://d2lvgg3v3hfg70.cloudfront.net/MD0008/11ecba2d_11cf_8577_a3bf_27808d2bab50_MD0008_00.jpg)
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
![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.[CO<sub>3</sub><sup>2-</sup>] = 2.5[Ca<sup>2+</sup>] + 4.1[Mg<sup>2+</sup>]<strong>Equation 1</strong>To 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% HNO<sub>3</sub> into 100 mL volumetric flasks. The calibration curves generated from the standard solutions are shown in Figures 1 and 2. <strong>Figure 1</strong> Calibration curve from the absorption of the Ca<sup>2+</sup> solution standards <strong>Figure 2</strong> Calibration curve from the absorption of the Mg<sup>2+</sup> solution standardsLanthanum(III) chloride (MM = 245.25) can be used to eliminate chemical interference in a sample by competitively binding to PO<sub>4</sub><sup>3−</sup>, 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% HNO<sub>3</sub>, and 9 mL of 0.1 M LaCl<sub>3</sub> 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.<strong>Table 1</strong> AA Spectroscopy Results for Reservoir Water Samples -How many La<sup>3+</sup> ions are present in each of the unknown samples? A) 1.5 × 10<sup>−27</sup> B) 5.4 × 10<sup>20</sup> C) 9.0 × 10<sup>20</sup> D) 6.0 × 10<sup>22</sup>](https://d2lvgg3v3hfg70.cloudfront.net/MD0008/11ecba2d_11cf_8578_a3bf_f13a744b20b5_MD0008_00.jpg)
-How many La3+ ions are present in each of the unknown samples?
A) 1.5 × 10−27
B) 5.4 × 1020
C) 9.0 × 1020
D) 6.0 × 1022
Correct Answer:
Verified
Correct Answer:
Verified
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