Multiple Choice
What is the molarity of a solution made by dissolving 10.0 g of sodium chloride (58.44 g/mol) in 750.0 mL of solution?
6. Chemical Quantities & Aqueous Reactions
Molarity
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What is the molarity of Fe(SCN)²⁺ in a solution with an absorbance of 0.243, prepared by mixing 2.5 mL of 0.200 M Fe(NO₃)₃, 0.4 mL of 0.002 M NaSCN, and 7.1 mL of 0.10 M HNO₃?
A
0.00004 M
B
0.00020 M
C
0.00016 M
D
0.00008 M
Verified step by step guidance1
Step 1: Understand the relationship between absorbance and concentration using Beer's Law, which is expressed as \( A = \varepsilon c l \), where \( A \) is the absorbance, \( \varepsilon \) is the molar absorptivity, \( c \) is the concentration, and \( l \) is the path length of the cuvette (usually 1 cm).
Step 2: Calculate the total volume of the solution by adding the volumes of the individual components: \( 2.5 \text{ mL} + 0.4 \text{ mL} + 7.1 \text{ mL} = 10 \text{ mL} \). Convert this total volume to liters for molarity calculations: \( 10 \text{ mL} = 0.010 \text{ L} \).
Step 3: Determine the initial moles of each reactant. For Fe(NO₃)₃: \( 2.5 \text{ mL} \times 0.200 \text{ M} = 0.0005 \text{ moles} \). For NaSCN: \( 0.4 \text{ mL} \times 0.002 \text{ M} = 0.0000008 \text{ moles} \).
Step 4: Assume that Fe(SCN)²⁺ is formed in a 1:1 ratio from Fe³⁺ and SCN⁻. The limiting reactant will determine the maximum amount of Fe(SCN)²⁺ formed. Compare the moles of Fe³⁺ and SCN⁻ to find the limiting reactant.
Step 5: Use the absorbance value and Beer's Law to find the molarity of Fe(SCN)²⁺. Rearrange Beer's Law to solve for concentration: \( c = \frac{A}{\varepsilon l} \). Use the given absorbance and known values for \( \varepsilon \) and \( l \) to calculate \( c \).
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