# Strengths of Acids and Bases

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## Strengths of Acids and Bases Answer Key

**Instructions:**
Complete the following questions using your knowledge of the strengths of acids and bases. For Questions #18-20, show your work as you answer each question.

1.

Which of the following would dissociate completely in water, yielding a positive hydrogen ion and an anion?

- hydrofluoric acid
- acetic acid
- hydrocyanic acid
- phosphoric acid
- hydrobromic acid

2.

Which equation correctly represents how ammonia dissociates when it reacts with water?

- [math]NH_3 rarr NH_4^+ + OH^-[/math]
- [math]NH_3 + H_2O rarr NH_4^+ + OH^-[/math]
- [math]NH_3 + H_2O rarr NH_2^"-" + H_3O^"+"[/math]
- Ammonia does not dissociate at all in water.

3.

Which of the ions would most likely be produced upon the dissociation of a strong base in water?

- [math]Cl^"-"[/math]
- [math]NH_4^+[/math]
- [math]Ba^"2+"[/math]
- [math]H^+[/math]
- [math]SO_4^"2-"[/math]

4.

How does the acid dissociation constant affect the strength of an acid?

- The larger the acid dissociation constant is, the stronger the acid.
- The larger the acid dissociation constant is, the weaker the acid.
- The acid dissociation constant has no effect on the acid strength.
- An increase in the acid dissociation constant can either increase or decrease acid strength.
- None of the above - there is no relationship.

5.

Which of the following correctly represents the expression for the acid dissociation constant of hydrocyanic acid?

- [math]K_a=[H^"+"][CN^"-"][/math]
- [math]K_a=[H_3O^"+"][CN^"-"][/math]
- [math]K_a={[H_3O^"+"][CN^"-"]}/{[HCN]}[/math]
- [math]K_a={[H_3O^"+"][CN^"-"]}/{[HCN][H_2O]}[/math]

6.

Which group contains ions that combine with hydroxide to form strong bases? (Select all that apply)

- Group 13
- Group 1
- Group 2
- Group 17
- Group 16
- A combination of Group 17 and 16

7.

Which of the following represents the correct base dissociation constant expression for aniline in water?

- [math]{[C_6N_5NH_3^+][OH^-]}/{[C_6N_5NH_2]}[/math]
- [math]{[C_6N_5NH_3^+][OH^-]}/{[C_6N_5NH_2][H_2O]}[/math]
- [math][C_6N_5NH^+][H_3O^+][/math]
- [math]{[C_6N_5NH^+][H_3O^+]}/{[OH^-]}[/math]

8.

Which of the following statements are correct, with regards to the strength of hydrochloric acid? (Select all that apply)

- Its acid dissociation constant is large
- Its acid dissociation constant is small
- Its pH depends on the concentration of the undissociated acid
- Its pH depends on the concentration of the hydronium ions
- The concentration of the hydronium ions equals that of the original acid

9.

Why should the initial molarity of hydrofluoric acid be included in the calculations of the acid dissociation constant? (Select all that apply)

- All of the acid ionizes
- The acid dissociation constant is larger in value
- The concentration of hydrofluoric acid affects the concentration of hydrogen ions
- Not all of the hydrofluoric acid ionizes
- None of the above

10.

How should the concentration of a VERY dilute solution of carbonic acid be determined?

- By finding the cubic root of the concentration of carbonic acid
- By finding the square root of the concentration of carbonic acid
- By taking into account the ions produced by water
- By ignoring the concentration of carbonic acid and focusing on the concentration of ions produced by water

11.

If the concentration of HCN is 0.50 M, what is the concentration of hydrogen ions in solution, presuming that [math]K_a=6.2 xx 10^-10[/math]?

- [math]3.52 xx 10^-5 M[/math]
- [math]1.55 xx 10^-10 M[/math]
- [math]3.1 xx 10^-10 M[/math]
- [math]1.76 xx 10^-5 M[/math]

12.

What is the concentration of hydrogen ions in solution, if the concentration of hydrobromic acid is 0.25 M?

- 0.50 M
- 0.25 M
- 0.125 M
- 0.0625 M

13.

What is the concentration of hydroxide ions in solution, if the concentration of calcium hydroxide is 0.26 M?

- 0.26 M
- 0.52 M
- 0.0676 M
- 0.72 M
- 0.27 M

14.

Which of the following would most likely have the greatest pOH value?

- 0.42 M potassium hydroxide
- 0.047 M calcium hydroxide
- 0.088 M hydrochloric acid
- 0.42 M nitric acid

15.

What would be the calculated pH value of 0.088 M HCl?

- 1.06
- 2.11
- 0.53
- 0.265

16.

What would be the calculated pH value of a [math]1.9 xx 10^-9 "M"[/math] hydrochloric acid solution?

- 8.72
- 6.99
- 4.36
- 13.98

17.

What is the pH value of 0.084 M calcium hydroxide?

- 1.07
- 0.77
- 13.23
- 12.92

18.

If the concentration of [math]NH_3[/math] is 0.091 M, what is the concentration of hydroxide ions in solution?

[math]"Note: Presume " K_b=1.8 xx 10^-5 " for " NH_3[/math]

[math]"Note: Presume " K_b=1.8 xx 10^-5 " for " NH_3[/math]

- [math]K_b={[NH_4^+][OH^-]}/{[NH_3]}=x^2/{0.091 M}=1.8 xx 10^-5[/math]

[math]x^2=1.638 xx 10^-6[/math]

[math]x=sqrt(1.638 xx 10^-6)=1.28 xx 10^-3 M[/math]

19.

The concentration of HBr is [math]1.6 xx 10^-8 M[/math]. Find its pOH value.

- [math][H_3O^+]=1.6 xx 10^-8 M + 1.0 xx 10^-7 M=1.16 xx 10^-7 M[/math]

[math]pH=-log[H_3O^+]=-log[1.16 xx 10^-7 M]=6.94[/math]

pOH=14-pH=14-6.94=7.06

20.

Determine the pH of 0.012 M acetic acid, presuming its acid dissociation constant is [math]1.76 xx 10^-5[/math].

- [math]K_a=1.76 xx 10^-5={[H_3O^+][CH_3COO^-]}/{[CH_3COOH]}={x^2}/{0.012 M}[/math]

[math]x^2=2.11 xx 10^-7[/math]

[math][H^+]=[CH_3COO^-]=4.6 xx 10^-4 M[/math]

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