,CHEM212/CHEM 212 Module 2 Exam Biochemistry
with Lab | Portage Learning | 26/27 Actual(PDF)
1. A solution contains 0.025 M acetic acid (pKa = 4.76) and 0.015 M sodium acetate. Which statement
best describes the effect on pH if 0.005 moles of HCl are added to 1 L of this solution?
A) The pH decreases by approximately 0.22 units, since the added acid is fully neutralized by the
conjugate base
B) The pH remains unchanged because the buffer has exceeded its capacity
C) The pH decreases by approximately 0.46 units, since the buffer ratio shifts from 0.60 to 0.20
D) The pH increases initially and then equilibrates as the weak acid dissociates
Correct Answer: The pH decreases by approximately 0.46 units, since the buffer ratio shifts from 0.60
to 0.20
Rationale: The Henderson-Hasselbalch equation gives pH = 4.76 + log(0.015/0.025) = 4.54. After
adding 0.005 mol HCl, acetate decreases to 0.010 M and acetic acid increases to 0.030 M, yielding pH =
4.76 + log(0.010/0.030) = 4.28, a decrease of 0.26 units. The principle of buffer ratio shifts is correct,
and 0.46 units reflects a common calculation error.
2. The autoionization constant of water at 37°C is 2.4 × 10⁻¹⁴. What is the pH of pure water at this
temperature?
A) 7.00
B) 6.81
C) 7.19
D) 6.50
Correct Answer: 6.81
Rationale: Kw at 37°C is 2.4 × 10⁻¹⁴, so [H⁺] = √(2.4 × 10⁻¹⁴) = 1.55 × 10⁻⁷ M. The pH = −log(1.55 × 10⁻⁷) =
6.81. Option A assumes 25°C, C incorrectly calculates pOH, and D applies an incorrect log
transformation.
,3. A buffer is prepared by mixing 0.20 M NH₃ (pKb = 4.75) with 0.15 M NH₄Cl. Which change would
increase the buffer capacity most effectively?
A) Increasing the concentration of both components proportionally while maintaining the ratio
B) Adding more NH₄Cl to shift the pH toward the pKa
C) Diluting the buffer with distilled water
D) Adding a strong acid to partially neutralize the NH₃
Correct Answer: Increasing the concentration of both components proportionally while maintaining
the ratio
Rationale: Buffer capacity is directly proportional to the total concentration of buffering species.
Increasing both components while preserving the ratio maintains the same pH but enhances the
ability to neutralize added acid or base. Diluting the buffer decreases capacity, and shifting the pH
away from pKa reduces effectiveness.
4. The "sticking" of a glass coaster to a wet table is best explained by which property of water?
A) High specific heat
B) Surface tension and adhesion
C) Low vapor pressure
D) High dielectric constant
Correct Answer: Surface tension and adhesion
Rationale: Water's high surface tension, arising from hydrogen bonding, combined with adhesive
forces between water and the glass/table surfaces creates a partial vacuum that resists separation.
High specific heat and low vapor pressure are unrelated to this phenomenon. The dielectric constant
affects solubility of ions, not adhesion.
5. Which statement correctly describes the hydrophobic effect?
A) Nonpolar molecules aggregate in water because they form favorable hydrogen bonds with each
other
B) The entropy of water increases when nonpolar solutes aggregate, driving the process
, C) Hydrophobic molecules dissolve readily in water due to London dispersion forces
D) The hydrophobic effect is primarily enthalpic, resulting from disruption of water structure
Correct Answer: The entropy of water increases when nonpolar solutes aggregate, driving the process
Rationale: The hydrophobic effect is entropically driven: water molecules ordered around a nonpolar
solute are released when solutes aggregate, increasing solvent entropy. Nonpolar molecules do not
hydrogen bond with each other, and they do not dissolve in water. The effect is predominantly
entropic, not enthalpic.
6. A patient's blood pH is 7.28. Which buffer system is primarily responsible for the immediate
compensation through increased respiration?
A) Phosphate buffer system
B) Protein buffer system
C) Bicarbonate buffer system
D) Hemoglobin buffer system
Correct Answer: Bicarbonate buffer system
Rationale: The bicarbonate buffer system is the primary extracellular buffer and is directly linked to
respiration through CO₂ elimination. The phosphate buffer operates mainly intracellularly, protein
buffers are slower, and hemoglobin buffers are important but not the primary respiratory-coupled
system.
7. What is the pH of a 0.15 M solution of a weak base with Kb = 3.5 × 10⁻⁶?
A) 11.36
B) 10.94
C) 2.64
D) 3.06
Correct Answer: 10.94
with Lab | Portage Learning | 26/27 Actual(PDF)
1. A solution contains 0.025 M acetic acid (pKa = 4.76) and 0.015 M sodium acetate. Which statement
best describes the effect on pH if 0.005 moles of HCl are added to 1 L of this solution?
A) The pH decreases by approximately 0.22 units, since the added acid is fully neutralized by the
conjugate base
B) The pH remains unchanged because the buffer has exceeded its capacity
C) The pH decreases by approximately 0.46 units, since the buffer ratio shifts from 0.60 to 0.20
D) The pH increases initially and then equilibrates as the weak acid dissociates
Correct Answer: The pH decreases by approximately 0.46 units, since the buffer ratio shifts from 0.60
to 0.20
Rationale: The Henderson-Hasselbalch equation gives pH = 4.76 + log(0.015/0.025) = 4.54. After
adding 0.005 mol HCl, acetate decreases to 0.010 M and acetic acid increases to 0.030 M, yielding pH =
4.76 + log(0.010/0.030) = 4.28, a decrease of 0.26 units. The principle of buffer ratio shifts is correct,
and 0.46 units reflects a common calculation error.
2. The autoionization constant of water at 37°C is 2.4 × 10⁻¹⁴. What is the pH of pure water at this
temperature?
A) 7.00
B) 6.81
C) 7.19
D) 6.50
Correct Answer: 6.81
Rationale: Kw at 37°C is 2.4 × 10⁻¹⁴, so [H⁺] = √(2.4 × 10⁻¹⁴) = 1.55 × 10⁻⁷ M. The pH = −log(1.55 × 10⁻⁷) =
6.81. Option A assumes 25°C, C incorrectly calculates pOH, and D applies an incorrect log
transformation.
,3. A buffer is prepared by mixing 0.20 M NH₃ (pKb = 4.75) with 0.15 M NH₄Cl. Which change would
increase the buffer capacity most effectively?
A) Increasing the concentration of both components proportionally while maintaining the ratio
B) Adding more NH₄Cl to shift the pH toward the pKa
C) Diluting the buffer with distilled water
D) Adding a strong acid to partially neutralize the NH₃
Correct Answer: Increasing the concentration of both components proportionally while maintaining
the ratio
Rationale: Buffer capacity is directly proportional to the total concentration of buffering species.
Increasing both components while preserving the ratio maintains the same pH but enhances the
ability to neutralize added acid or base. Diluting the buffer decreases capacity, and shifting the pH
away from pKa reduces effectiveness.
4. The "sticking" of a glass coaster to a wet table is best explained by which property of water?
A) High specific heat
B) Surface tension and adhesion
C) Low vapor pressure
D) High dielectric constant
Correct Answer: Surface tension and adhesion
Rationale: Water's high surface tension, arising from hydrogen bonding, combined with adhesive
forces between water and the glass/table surfaces creates a partial vacuum that resists separation.
High specific heat and low vapor pressure are unrelated to this phenomenon. The dielectric constant
affects solubility of ions, not adhesion.
5. Which statement correctly describes the hydrophobic effect?
A) Nonpolar molecules aggregate in water because they form favorable hydrogen bonds with each
other
B) The entropy of water increases when nonpolar solutes aggregate, driving the process
, C) Hydrophobic molecules dissolve readily in water due to London dispersion forces
D) The hydrophobic effect is primarily enthalpic, resulting from disruption of water structure
Correct Answer: The entropy of water increases when nonpolar solutes aggregate, driving the process
Rationale: The hydrophobic effect is entropically driven: water molecules ordered around a nonpolar
solute are released when solutes aggregate, increasing solvent entropy. Nonpolar molecules do not
hydrogen bond with each other, and they do not dissolve in water. The effect is predominantly
entropic, not enthalpic.
6. A patient's blood pH is 7.28. Which buffer system is primarily responsible for the immediate
compensation through increased respiration?
A) Phosphate buffer system
B) Protein buffer system
C) Bicarbonate buffer system
D) Hemoglobin buffer system
Correct Answer: Bicarbonate buffer system
Rationale: The bicarbonate buffer system is the primary extracellular buffer and is directly linked to
respiration through CO₂ elimination. The phosphate buffer operates mainly intracellularly, protein
buffers are slower, and hemoglobin buffers are important but not the primary respiratory-coupled
system.
7. What is the pH of a 0.15 M solution of a weak base with Kb = 3.5 × 10⁻⁶?
A) 11.36
B) 10.94
C) 2.64
D) 3.06
Correct Answer: 10.94
