Q: Given the reaction $$\mathrm{Fe$$^{3+}$$ (aq) + SCN^- (aq) \rightleftharpoons $$FeSCN^{2+}$$ (aq)}$$, which of the following is the correct equilibrium constant expression ($$K_c$$)?

$$K_c = \dfrac{[$$FeSCN^{2+}$$]_{eq}}{[Fe$$^{3+}$$]_{eq} \times [SCN^-]_{eq}}$$

Q: If the absorbance of a solution is measured to be 0.40 at 447 nm, and the calibration curve equation is $$A = 3500C$$, what is the concentration of $$FeSCN^{2+} in $$the solution?

$$1.14 \times 10$$^{-4}$$ \ \mathrm{M}$$

Q: A student prepares an equilibrium mixture with initial concentrations $$[Fe$$^{3+}$$]_i = 1.00 \times 10$$^{-3}$$ \ \mathrm{M}$$ and $$[SCN^-]_i = 1.00 \times 10$$^{-3}$$ \ \mathrm{M}. At $$equilibrium, $$[$$FeSCN^{2+}$$]_{eq} = 3.00 \times 10$$^{-4}$$ \ \mathrm{M}. $$What is $$[Fe$$^{3+}$$]_{eq}$$?

$$7.00 \times 10$$^{-4}$$ \ \mathrm{M}$$

Question 1

Which of the following best describes the equilibrium state in a chemical reaction?

Accepted Answer

The forward and reverse reactions occur at equal rates, resulting in constant concentrations of reactants and products.

Question 2

Given the reaction $$\mathrm{Fe$$^{3+}$$ (aq) + SCN^- (aq) \rightleftharpoons $$FeSCN^{2+}$$ (aq)}$$, which of the following is the correct equilibrium constant expression ($$K_c$$)?

Accepted Answer

$$K_c = \dfrac{[$$FeSCN^{2+}$$]_{eq}}{[Fe$$^{3+}$$]_{eq} 	imes [SCN^-]_{eq}}$$

Question 3

A solution containing $$FeSCN^{2+}$$ appears orange. According to the color wheel, which wavelength range should be used to measure its absorbance most effectively?

Accepted Answer

450–480 nm (blue)

Question 4

Which equation correctly represents Beer’s Law as used in spectrophotometry?

Accepted Answer

$$A = ELC$$

Question 5

Why is it important to use the same spectrophotometer for all absorbance measurements in this experiment?

Accepted Answer

Different spectrophotometers may have slight calibration differences, affecting the accuracy and consistency of absorbance readings.

Question 6

If the absorbance of a solution is measured to be 0.40 at 447 nm, and the calibration curve equation is $$A = 3500C$$, what is the concentration of $$FeSCN^{2+} in $$the solution?

Accepted Answer

$$1.14 	imes 10$$^{-4}$$ \ \mathrm{M}$$

Question 7

In preparing the standard solutions for the calibration curve, why is a large excess of $$Fe^{3+}$$ used?

Accepted Answer

To ensure that all $$SCN^-$$ reacts to form $$FeSCN^{2+}$$, making $$SCN^-$$ the limiting reactant.

Question 8

A student prepares an equilibrium mixture with initial concentrations $$[Fe$$^{3+}$$]_i = 1.00 	imes 10$$^{-3}$$ \ \mathrm{M}$$ and $$[SCN^-]_i = 1.00 	imes 10$$^{-3}$$ \ \mathrm{M}. At $$equilibrium, $$[$$FeSCN^{2+}$$]_{eq} = 3.00 	imes 10$$^{-4}$$ \ \mathrm{M}. $$What is $$[Fe$$^{3+}$$]_{eq}$$?

Accepted Answer

$$7.00 	imes 10$$^{-4}$$ \ \mathrm{M}$$

Question 9

Which of the following best explains why the absorbance of a solution is directly proportional to the concentration of the colored species?

Accepted Answer

More molecules of the colored species absorb more light, reducing the intensity of transmitted light.

Question 10

A student accidentally uses a dirty cuvette when measuring absorbance. What is the most likely effect on the measured absorbance value?

Accepted Answer

The measured absorbance will be artificially high due to scattering and absorption by residues.

Question 11

If the equilibrium constant $$K_c$$ for the reaction $$Fe^{3+} + $$SCN^- \rightleftharpoons $$FeSCN^{2+} is $$found to be much greater than 1, what does this indicate about the position of equilibrium?

Accepted Answer

The equilibrium favors the formation of $$FeSCN^{2+} ($$products).

Question 12

Which safety precaution is most important when handling $$Fe(NO_3$)_3$$ solutions in this experiment?

Accepted Answer

Wear safety goggles, a lab coat, and nitrile gloves to protect against corrosive nitric acid.

Question 13

A student mixes 5.00 mL of $$2.00 	imes 10$$^{-3}$$ \ \mathrm{M} \ Fe(NO_3$)_3$$ with 3.00 mL of $$2.00 	imes 10$$^{-3}$$ \ \mathrm{M} \ KSCN$$ and 2.00 mL of water. What is the initial concentration of $$Fe^{3+} in $$the mixture?

Accepted Answer

$$1.00 	imes 10$$^{-3}$$ \ \mathrm{M}$$

Question 14

Which step is essential before using a pipet with a new chemical solution?

Accepted Answer

Rinse the pipet first with deionized water, then with 1–2 mL of the new chemical solution.

Question 15

After completing the experiment, what should you do with all chemical waste and rinse solutions?

Accepted Answer

Collect all waste in a designated beaker and dispose of it in the provided waste container.

Question 16

If a student prepares a calibration curve and finds that the absorbance does not increase linearly with concentration, what is the most likely cause?

Accepted Answer

The concentration range used is too high, causing deviations from Beer’s Law.

Question 17

Which of the following best describes the purpose of using a 'blank' solution in spectrophotometry?

Accepted Answer

To zero the spectrophotometer by accounting for absorbance due to the solvent and cuvette.

Equilibrium Constant for the Formation of a Complex Ion: FeSCN²⁺

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