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

-What is the molar concentration of Cl⁻ ions in a solution prepared by dissolving 0.1 moles of lanthanum(III) chloride in 1 L of water?

Question

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

-What is the molar concentration of Cl⁻ ions in a solution prepared by dissolving 0.1 moles of lanthanum(III) chloride in 1 L of water?

Quizplus · Accepted Answer

11ecba2d_11cf_ac89_a3bf_8356043ff3a5_MD0008_00 The mole ratio is the whole number proportion of each chemical species in a balanced chemical equation or molecular formula. If the amount of a chemical substance is given in something other than moles (eg, grams), then this amount must be converted to moles first to relate the various species proportions. Molarity is defined as moles of solute per liter of solution. Because molarity expresses concentration in terms of moles, the mole ratio of the solute ions can be multiplied by the concentration of the LaCl₃ in solution to find the concentration of individual ions in solution. The concentration of LaCl₃ is 0.1 M, and based on the molecular formula, there are 3 moles of Cl⁻ ions in every 1 mole of LaCl₃. The concentration of LaCl₃ can be related to the concentration of Cl⁻ ions by using this mole ratio as a conversion factor: 11ecba2d_11cf_ac8a_a3bf_757b631fe0f2_MD0008_00 Therefore, the concentration of Cl⁻ ions must be 0.3 M. (Choice A) 0.03 M is obtained by dividing rather than multiplying the concentration of LaCl₃ by 3 moles Cl⁻. This operation does not cancel out the moles of LaCl₃, and therefore provides incorrect units. (Choice B) 0.1 M is the concentration of LaCl₃. The concentration of Cl⁻ must be larger than the concentration of LaCl₃ because there are 3 Cl⁻ ions for every La³⁺. (Choice D) 30 M is obtained if the mole ratio of Cl⁻ to LaCl₃ is divided by, instead of multiplied by, the concentration of LaCl₃. This will not cancel out the moles of LaCl₃, resulting in an answer with incorrect units. Educational objective: The mole ratio is the whole number proportion of each chemical species in a balanced chemical equation. Molarity is the number of moles of solute per liter of solution. Because molarity expresses concentration in terms of moles, a mole ratio can be used to relate the concentration (molarity) of one species to the concentration of another species in solution.