Q: Given $$[\mathrm{H_3O^+}] = 7.5 \times 10$$^{-5}$$\ \mathrm{M} at$$ $$25\ ^\circ\mathrm{C}$$, calculate $$[\mathrm{OH^-}]$$ and classify the solution as acidic, basic, or neutral.

$$[\mathrm{OH^-}] = 1.3 \times 10$$^{-10}$$\ \mathrm{M}$$,\ \text{acidic}$

Q: A $$0.100\ \mathrm{M}$$ solution of a weak acid $$HA$$ has $$[\mathrm{H_3O^+}] = 5.6 \times 10$$^{-5}$$\ \mathrm{M}. $$What is the value of $$K_a$$ for this acid?

$$K_a = \dfrac{(5.6 \times 10$$^{-5}$$)^2$$}{0.100} = 3.1 \times $$10^{-8}$

Q: Which of the following statements is true regarding the $$x is $$small approximation in weak acid equilibrium calculations?

The approximation is valid if $$\dfrac{x}{\text{initial concentration}} \u003c 0.05$$

Q: A $$0.100\ \mathrm{M}$$ solution of $$\mathrm{NH_3$}$$ has a $$K_b = 1.76 \times 10$$^{-5}$$. What is the equilibrium concentration of $$\mathrm{OH^-}$$?

$$1.33 \times 10$$^{-3}$$\ \mathrm{M}$$

Q: Which of the following is the correct formula for calculating percent ionization of a weak acid?

$$\dfrac{[\mathrm{H_3O^+}]_{\text{equil}}}{[\mathrm{HA}]_{\text{init}}} \times 100\%$$

Question 1

In the reaction $$\mathrm{NH_3$(aq) + H_2O(l) \rightleftharpoons NH_4$^+(aq) + OH^-(aq)}$$, which species acts as the Brønsted–Lowry base?

Accepted Answer

$$\mathrm{NH_3$}$$

Question 2

Given $$[\mathrm{H_3O^+}] = 7.5 	imes 10$$^{-5}$$\ \mathrm{M} at$$ $$25\ ^\circ\mathrm{C}$$, calculate $$[\mathrm{OH^-}]$$ and classify the solution as acidic, basic, or neutral.

Accepted Answer

$$[\mathrm{OH^-}] = 1.3 	imes 10$$^{-10}$$\ \mathrm{M}$$,\ 	ext{acidic}$

Question 3

Which of the following is the correct expression for the acid dissociation constant $$K_a$$ for the ionization of HCN in water?

Accepted Answer

$$K_a = \dfrac{[\mathrm{H_3O^+}][\mathrm{CN^-}]}{[\mathrm{HCN}]}$$

Question 4

A $$0.100\ \mathrm{M}$$ solution of a weak acid $$HA$$ has $$[\mathrm{H_3O^+}] = 5.6 	imes 10$$^{-5}$$\ \mathrm{M}. $$What is the value of $$K_a$$ for this acid?

Accepted Answer

$$K_a = \dfrac{(5.6 	imes 10$$^{-5}$$)^2$$}{0.100} = 3.1 	imes $$10^{-8}$

Question 5

Which of the following anions is pH-neutral in aqueous solution?

Accepted Answer

$$\mathrm{NO_3$^-}$$

Question 6

Calculate the pH of a solution with $$[\mathrm{H_3O^+}] = 1.8 	imes 10$$^{-4}$$\ \mathrm{M} at$$ $$25\ ^\circ\mathrm{C}.$$

Accepted Answer

$$\mathrm{pH} = 3.74$$

Question 7

Which of the following statements is true regarding the $$x is $$small approximation in weak acid equilibrium calculations?

Accepted Answer

The approximation is valid if $$\dfrac{x}{	ext{initial concentration}} \u003c 0.05$$

Question 8

A $$0.100\ \mathrm{M}$$ solution of $$\mathrm{NH_3$}$$ has a $$K_b = 1.76 	imes 10$$^{-5}$$. What is the equilibrium concentration of $$\mathrm{OH^-}$$?

Accepted Answer

$$1.33 	imes 10$$^{-3}$$\ \mathrm{M}$$

Question 9

Which salt will produce a basic solution when dissolved in water?

Accepted Answer

$$\mathrm{$$NaCHO_2$$}$$

Question 10

A $$0.0100\ \mathrm{M}$$ solution of $$\mathrm{H_2SO_4} is $$prepared. Why must the second ionization step be considered in the pH calculation?

Accepted Answer

Because the second ionization step produces a significant amount of $$\mathrm{H_3O^+} at $$low concentrations

Question 11

Which of the following cations is classified as a weak acid in aqueous solution?

Accepted Answer

$$\mathrm{C_5H_5NH^+}$$

Question 12

A $$0.100\ \mathrm{M}$$ solution of $$\mathrm{$$NaCHO_2$$} is $$prepared. Which ion is responsible for the basicity of the solution?

Accepted Answer

$$\mathrm{$$CHO_2$$^-}$$

Question 13

Which of the following is the correct formula for calculating percent ionization of a weak acid?

Accepted Answer

$$\dfrac{[\mathrm{H_3O^+}]_{	ext{equil}}}{[\mathrm{HA}]_{	ext{init}}} 	imes 100\%$$

Question 14

Which of the following statements best explains why $$\mathrm{NO_2$^-} is a $$weak base in water?

Accepted Answer

It is the conjugate base of a weak acid

Question 15

A $$0.225\ \mathrm{M}$$ solution of KOH is prepared. What is the pH of the solution at $$25\ ^\circ\mathrm{C}$$?

Accepted Answer

$$\mathrm{pH} = 13.352$$

Question 16

Which of the following best describes the role of water in the reaction $$\mathrm{H_2SO_4(aq) + H_2O(l) \rightarrow $$HSO_4$$^-(aq) + H_3O^+(aq)}$$?

Accepted Answer

Water acts as a Brønsted–Lowry base

Question 17

A $$0.100\ \mathrm{M}$$ solution of $$\mathrm{$$HClO_2$$}$$ has $$K_a = 0.011. $$Why is the $$x is $$small approximation not valid in this case?

Accepted Answer

Because $$K_a is $$not small relative to the initial concentration

Question 18

Which of the following best describes the pH of a solution formed by dissolving $$\mathrm{$$AlBr_3$$} in $$water?

Accepted Answer

The solution is acidic because $$\mathrm{Al$$^{3+}$$} is a $$weak acid

Acids, Bases, and Equilibrium: Brønsted–Lowry Theory and Calculations

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