Q: If the absorbance of a solution at 470 nm is 0.500 and the path length is 1.00 cm, with $$\varepsilon = 4700\ \text{L mol}^{-1}\text{cm}^{-1}$$, what is the concentration of $$\ce{Fe($$SCN)^{2+}$$}$$?

$$1.06 \times 10$$^{-4}$$\ \text{M}$$

Question 1

A student prepares a solution by dissolving 5.0 g of lithium hydroxide (LiOH) in 200 mL of water. What is the pH of the resulting solution? (Molar mass of LiOH = 24 g/mol)

Accepted Answer

$$	ext{pH} = 13.0$$

Question 2

The equilibrium constant expression for the reaction $$\ce{Fe$$^{3+}$$ + SCN^-  Fe($$SCN)^{2+}$$} is$$:

Accepted Answer

$$K = \frac{[\ce{Fe($$SCN)^{2+}$$}]}{[\ce{Fe$$^{3+}$$}][\ce{SCN^-}]}$$

Question 3

A typical aspirin tablet contains 325 mg of aspirin (C$$_9H$_8O$_4). $$Aspirin is a monoprotic acid with $$K_a = 3.0 	imes 10$$^{-4}$$. If you dissolve four tablets in 0.40 L of water, what is the pH? (Molar mass of aspirin = 180 g/mol)

Accepted Answer

$$	ext{pH} \approx 2.1$$

Question 4

The pH of a 0.040 M hydrofluorous acid (HFO$$_2) $$solution is 5.06. What is the acid dissociation constant ($$K_a) $$for this acid?

Accepted Answer

$$K_a = 2.3 	imes 10$$^{-8}$$

Question 5

Is hydrofluorous acid (HFO$$_2) a $$strong acid or a weak acid? Explain your answer based on the calculated $$K_a$$ value.

Accepted Answer

It is a weak acid because $$K_a is $$much less than 1.

Question 6

If you mix 25 mL of 12 M HCl with 25 mL of 12 M NaOH, what is the pH of the resulting solution?

Accepted Answer

$$	ext{pH} = 7$$

Question 7

In the experiment to determine the equilibrium constant for $$\ce{Fe$$^{3+}$$ + SCN^-  Fe($$SCN)^{2+}$$}$$, why is it important to use the absorbance at 470 nm rather than at 400 nm or 500 nm?

Accepted Answer

470 nm provides maximum sensitivity for $$\ce{Fe($$SCN)^{2+}$$}$$ detection.

Question 8

Given the reaction $$\ce{2SO_2(g) + O_2$(g)  2SO_3(g)}$$ with $$K_c = 4.0 at a $$certain temperature, what will happen to the equilibrium if more $$\ce{SO_2$} is $$added?

Accepted Answer

The equilibrium will shift to the right, forming more $$\ce{SO_3$}.$$

Question 9

A solution is prepared by mixing 0.10 mol of $$\ce{H_2$}$$, 0.20 mol of $$\ce{I_2$}$$, and 0.30 mol of $$\ce{HI} in a 1.0 L $$flask. For the reaction $$\ce{H_2$(g) + I_2$(g)  2HI(g)}$$, $$K_c = 50 at 448$$°C. What is the direction of the shift to reach equilibrium?

Accepted Answer

The reaction will shift to the right to form more $$\ce{HI}.$$

Question 10

Which of the following best describes the relationship between absorbance (A) and concentration (c) according to Beer's Law?

Accepted Answer

$$A = \varepsilon c l$$

Question 11

If the absorbance of a solution at 470 nm is 0.500 and the path length is 1.00 cm, with $$\varepsilon = 4700\ 	ext{L mol}^{-1}	ext{cm}^{-1}$$, what is the concentration of $$\ce{Fe($$SCN)^{2+}$$}$$?

Accepted Answer

$$1.06 	imes 10$$^{-4}$$\ 	ext{M}$$

Question 12

A student calculates the mean equilibrium constant $$K$$ for the reaction $$\ce{Fe$$^{3+}$$ + SCN^-  Fe($$SCN)^{2+}$$}$$ from six trials. Which of the following best describes the purpose of calculating the mean $$K$$?

Accepted Answer

To minimize the effect of random errors and obtain a more accurate value.

Question 13

Which of the following statements about equilibrium is correct?

Accepted Answer

At equilibrium, the rates of the forward and reverse reactions are equal.

Question 14

If the absorbance at 400 nm or 500 nm is used instead of 470 nm in the experiment, would the same value of $$K be $$obtained? Why or why not?

Accepted Answer

Yes, as long as Beer's Law is obeyed and the same species is measured.

General Chemistry Study Guidance: Equilibrium, Acids & Bases, and Solution Calculations

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