Question 1

Which of the following statements best explains why acetic acid ($$\mathrm{CH_3COOH}$$, $$pK_a = 4.8) is a $$stronger acid than ethanol ($$\mathrm{CH_3CH_2OH}$$, $$pK_a = 15.9$$)?

Accepted Answer

The conjugate base of acetic acid is stabilized by resonance, while the conjugate base of ethanol is not.

Question 2

Given the following reaction, which direction will the equilibrium favor?

$$\mathrm{CH_3CH_2OH + CH_3NH_2 \rightleftharpoons CH_3CH_2O^- + CH_3NH_3^+}$$

Given $$pK_a$$ values: $$\mathrm{CH_3CH_2OH}$$ ($$15.9$$), $$\mathrm{CH_3NH_3^+}$$ ($$10.7)$$

Accepted Answer

Toward the reactants, because $$\mathrm{CH_3CH_2O^-} is a $$stronger base than $$\mathrm{CH_3NH_2}.$$

Question 3

Which of the following species is the most stable conjugate base?

$$\mathrm{CH_3$^-}$$, $$\mathrm{NH_2$^-}$$, $$\mathrm{HO^-}$$, $$\mathrm{F^-}$$

Accepted Answer

$$\mathrm{F^-}$$

Question 4

Which of the following factors most significantly increases the acidity of hydrogen halides as you move down the group in the periodic table?

Accepted Answer

Increasing atomic size of the halogen atom, which stabilizes the conjugate base.

Question 5

Rank the following compounds in order of increasing acidity: $$\mathrm{CH_4$}$$, $$\mathrm{NH_3$}$$, $$\mathrm{H_2O}$$, $$\mathrm{HF}.$$

Accepted Answer

$$\mathrm{CH_4$} \u003c \mathrm{NH_3$} \u003c \mathrm{H_2O} \u003c \mathrm{HF}$$

Question 6

Which of the following statements about the effect of hybridization on acidity is correct?

Accepted Answer

An sp-hybridized carbon is more electronegative and thus increases acidity.

Question 7

Which of the following carboxylic acids will have the lowest pKa?

A. Acetic acid ($$\mathrm{CH_3COOH})
B. $$Chloroacetic acid ($$\mathrm{ClCH_2COOH})
C. $$Dichloroacetic acid ($$\mathrm{Cl_2CHCOOH})
D. $$Trichloroacetic acid ($$\mathrm{Cl_3CCOOH})$$

Accepted Answer

Trichloroacetic acid ($$\mathrm{Cl_3CCOOH})$$

Question 8

At which pH will the majority of a carboxylic acid ($$pK_a = 4.8) $$exist in its deprotonated form ($$\mathrm{RCOO^-}$$)?

Accepted Answer

pH = 7

Question 9

Which of the following best describes the effect of an electron-withdrawing group (EWG) on the acidity of a carboxylic acid?

Accepted Answer

It increases the acidity by stabilizing the conjugate base.

Question 10

Which of the following is NOT stabilized by resonance?

Accepted Answer

Methoxide ion ($$\mathrm{CH_3O^-})$$

Question 11

Which of the following statements is true regarding the relationship between acid strength and conjugate base strength?

Accepted Answer

The stronger the acid, the weaker its conjugate base.

Question 12

Which of the following best explains why $$\mathrm{HI} is a $$stronger acid than $$\mathrm{HF}$$?

Accepted Answer

$$\mathrm{I^-} is $$more stable than $$\mathrm{F^-}$$ due to its larger size, which better disperses the negative charge.

Question 13

Which of the following is the correct order of increasing stability for the conjugate bases of the hydrogen halides?

Accepted Answer

$$\mathrm{F^-} \u003c \mathrm{Cl^-} \u003c \mathrm{Br^-} \u003c \mathrm{I^-}$$

Question 14

Which of the following best describes the predominant form of an amine ($$pK_a = 10.7) at pH 1$$?

Accepted Answer

Mostly in the protonated form ($$\mathrm{$$RNH_3$$^+})$$

Question 15

Which of the following is the correct relationship between $$K_a$$ and $$pK_a$$?

Accepted Answer

$$pK_a = -\log_{10} K_a$$

Question 16

Which of the following statements about the effect of solution pH on the protonation state of a compound is correct?

Accepted Answer

When pH \u003c pK_a, the acid is mostly protonated.

Question 17

Which of the following best explains why methyl alcohol ($$\mathrm{CH_3OH}$$, $$pK_a = 15) is a $$stronger acid than methyl amine ($$\mathrm{CH_3NH_2}$$, $$pK_a = 40$$)?

Accepted Answer

Oxygen is more electronegative than nitrogen, stabilizing the negative charge on the conjugate base.

Question 18

How many times more acidic is ethyne ($$pK_a = 25) $$compared to ethene ($$pK_a = 44$$)?

Use the formula $$\mathrm{	ext{Acidity~difference} = 10$$^{(pK_{a,2}$$ - pK_{a,1})}}.$$

Accepted Answer

$$10^{19}$$ times more acidic

Acid-Base Reactions in Organic Chemistry: Principles and Applications

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