Question 1

Which of the following statements best describes the significance of pKa values in biochemistry?

Accepted Answer

pKa values reflect the pH at which a functional group is half protonated and half deprotonated.

Question 2

Given the following pKa values: $$\mathrm{pKa}_1 = 9.17$$ and $$\mathrm{pKa}_2 = 1.82$$, which of the following statements is correct regarding the acid-base properties of the molecule?

Accepted Answer

At pH 7, the group with $$\mathrm{pKa}_2$$ will be mostly deprotonated, while the group with $$\mathrm{pKa}_1$$ will be mostly protonated.

Question 3

A biochemist is studying a molecule with $$\mathrm{pKa}_1 = 9.17$$ and $$\mathrm{pKa}_2 = 1.82. If $$the solution pH is adjusted to 1.0, what is the predominant form of the molecule?

Accepted Answer

Both groups are protonated.

Question 4

Which equation is used to relate the pH, pKa, and the ratio of protonated to deprotonated forms of a weak acid?

Accepted Answer

$$pH = pKa + \log_{10}\left(\dfrac{[A^-]}{[HA]}\right)$$

Question 5

A patient’s blood pH drops to 6.8. Considering the Henderson-Hasselbalch equation, what is the likely effect on the ratio of bicarbonate ($$\mathrm{HCO}_3^-) to $$carbonic acid ($$\mathrm{H}_2\mathrm{CO}_3) in $$the blood?

Accepted Answer

The ratio decreases, favoring carbonic acid.

Question 6

If a molecule has two ionizable groups with $$\mathrm{pKa}_1 = 1.82$$ and $$\mathrm{pKa}_2 = 9.17$$, what is the isoelectric point ($$pI) of $$the molecule?

Accepted Answer

$$pI = \dfrac{1.82 + 9.17}{2} = 5.495$$

Question 7

Which of the following best explains why proteins are sensitive to changes in pH?

Accepted Answer

Changes in pH can alter the ionization state of amino acid side chains, affecting protein structure and function.

Question 8

A solution contains a weak acid with $$\mathrm{pKa} = 6.0. If $$the concentration of the acid ($$[HA]) is 0.1 M $$and the concentration of its conjugate base ($$[A^-]) is 1.0 M$$, what is the pH of the solution?

Accepted Answer

$$pH = 6.0 + \log_{10}(10)$$

Question 9

Which of the following scenarios would result in a buffer system being most effective?

Accepted Answer

When the pH is close to the pKa of the buffer.

Question 10

A researcher is preparing a buffer with $$\mathrm{pKa} = 7.0. $$She wants the buffer to maintain a pH of 7.0. What should be the ratio of $$[A^-] to$$ $$[HA]$$?

Accepted Answer

The ratio should be 1:1.

Question 11

Which of the following is a consequence of a buffer system in blood failing to maintain pH?

Accepted Answer

Proteins may denature, leading to loss of function.

Question 12

A solution contains a weak acid and its conjugate base. If the pH is 2 units above the pKa, what is the ratio of $$[A^-] to$$ $$[HA]$$?

Accepted Answer

$$[A^-]/[HA] = 100$$

Question 13

Which of the following best describes the role of buffers in biological systems?

Accepted Answer

Buffers maintain a stable pH by neutralizing added acids or bases.

Question 14

A patient is suffering from metabolic acidosis. Which buffer system in the blood is primarily responsible for compensating this condition?

Accepted Answer

Bicarbonate buffer system

Question 15

If the pH of a solution is equal to the pKa of the acid present, what is the ratio of the concentrations of the conjugate base to the acid?

Accepted Answer

The ratio is 1:1.

Question 16

A biochemist adds a strong acid to a buffer solution. What is the expected effect on the pH compared to adding the same amount of acid to pure water?

Accepted Answer

The pH of the buffer solution will decrease slightly, much less than pure water.

Question 17

Which of the following is the correct formula for calculating the isoelectric point ($$pI) of a $$molecule with two ionizable groups?

Accepted Answer

$$pI = \dfrac{$$pKa_1$$ + $$pKa_2$$}{2}$$

Question 18

A solution contains a weak acid with $$\mathrm{pKa} = 6.0. If $$the pH is 5.0, what is the ratio of $$[A^-] to$$ $$[HA]$$?

Accepted Answer

$$[A^-]/[HA] = 0.1$$

Acid-Base Properties of Amino Acids: pKa Values and Ionization

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