Acid Base - Level 2

Question 1

At what pH will a functional group with a pKa of 4.0 be mostly deprotonated?
a) pH 2.0
b) pH 3.5
c) pH 4.0
d) pH 6.0

Answer 1

Answer: d) pH 6.0

Explanation: A group becomes deprotonated when pH > pKa. At pH 6.0, the environment is basic enough to pull the proton off a group with pKa 4.0.

Question 2

When pH < pKa, a group is mostly ______________.
a) Protonated
b) Deprotonated

Answer 2

Answer: a) Protonated

Explanation: At pH < pKa, the environment is more acidic, so the group tends to hold onto its proton and remain protonated.

Question 3

A molecule has a carboxylic acid group (pKa = 4.5). At pH 7, what is the likely charge on this group?
a) Neutral
b) Positive
c) Negative
d) Amphipathic

Answer 3

Answer: c) Negative

Explanation: When pH > pKa, the carboxyl group loses its proton and becomes negatively charged (–COO⁻)

Question 4

Why are low pKa acids more ionized at high pH?
a) Because high pH favors protonation
b) Because low pKa acids cannot lose protons
c) Because high pH environments encourage acids to lose protons
d) Because pKa does not influence ionization

Answer 4

Answer: c) Because high pH environments encourage acids to lose protons

Explanation: Low pKa acids give up protons easily. At high pH, the environment is basic and pulls the proton off, increasing ionization.

Question 5

Why does a low pKa indicate a strong acid?
a) It reflects high proton donation tendency
b) It indicates more hydrogen bonding
c) It means the acid is less reactive
d) It shows low electronegativity

Answer 5

Answer: a) It reflects high proton donation tendency

Explanation: pKa is the negative log of Ka. A low pKa means a high Ka, which indicates the acid readily donates protons — a hallmark of a strong acid.

Question 6

Which statement explains why a strong acid has a high Ka?
a) It absorbs protons readily
b) It resists ionization
c) It dissociates completely in solution
d) It forms strong conjugate bases

Answer 6

Answer: c) It dissociates completely in solution

Explanation: Strong acids dissociate almost completely, releasing more H⁺ ions. This increases the value of Ka.

Question 7

What determines whether a functional group is protonated or deprotonated at a given pH?
a) The size of the molecule
b) The difference between pH and pKa
c) The number of hydrogen bonds
d) The concentration of buffer salts

Answer 7

Answer: b) The difference between pH and pKa

Explanation: When pH < pKa, the group is protonated; when pH > pKa, it is deprotonated. The relative values of pH and pKa determine the ionization state.

Question 8

At a pH below its pKa, what is the likely protonation state of a carboxylic acid group (–COOH)?
a) Ionized and negatively charged
b) Deprotonated and neutral
c) Protonated and neutral
d) Protonated and positively charged

Answer 8

Answer: c) Protonated and neutral

Explanation: At pH < pKa, the environment favors protonation. Carboxylic acids in this range tend to hold onto their proton, remaining neutral (–COOH).

Question 9

Which of the following functional groups is most likely to be ionized at pH 7?
a) A group with pKa = 2
b) A group with pKa = 5
c) A group with pKa = 7
d) A group with pKa = 10

Answer 9

Answer: a) A group with pKa = 2

Explanation: If pH is higher than the pKa, the group is deprotonated (ionized). The greater the difference (pH – pKa), the more ionized the group is.

Question 10

If the pKa of an amino group is 9.0, what will be its protonation state at physiological pH (7.4)?
a) Fully deprotonated and neutral
b) Mostly protonated and positively charged
c) Completely ionized and negative
d) Uncharged and hydrophobic

Answer 10

Answer: b) Mostly protonated and positively charged

Explanation: Since pH < pKa, the amino group will be mostly protonated (–NH₃⁺), contributing a positive charge.