CHEM 103 | CHEM103 Exam 4: General Chemistry
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. According to the Brønsted-Lowry theory, what is the definition of a base?
A. A substance that produces OH- ions in aqueous solution.
B. A substance that acts as a proton (H+) acceptor.
C. A substance that acts as an electron pair donor.
D. A substance that produces H+ ions in aqueous solution.
Correct Answer: B
Expert Explanation: The Brønsted-Lowry theory specifically defines a base as any species
capable of accepting a proton. This definition focuses on the transfer of H+ ions rather than
just the production of hydroxide ions in water. For example, ammonia acts as a base by
accepting a proton to form ammonium. Unlike the Arrhenius definition, this allows for base
behavior in non-aqueous environments. This concept is fundamental to understanding
acid-base conjugate pairs in equilibrium.
2. What is the pH of a 0.025 M solution of Hydrochloric Acid (HCl)?
A. 12.40
B. 1.40
C. 2.50
D. 1.60
Correct Answer: D
Expert Explanation: Hydrochloric acid is a strong acid that dissociates completely in
water to provide H+ ions. The pH is calculated using the formula pH = -log[H+]. Inserting
0.025 M into the equation gives -log(0.025), which equals approximately 1.60. Since HCl is
monoprotic, the concentration of the acid is equal to the concentration of the hydronium
ions. Understanding this logarithmic scale is essential for quantifying acidity in chemical
reactions.
3. Which of the following represents the conjugate base of the dihydrogen phosphate ion
(H2PO4-)?
A. HPO4^2-
B. H3PO4
C. PO4^3-
,D. OH-
Correct Answer: A
Expert Explanation: A conjugate base is formed when an acid loses a single proton (H+).
For H2PO4-, removing one H+ results in the formation of HPO4^2-. This relationship is part
of a series of equilibria for the polyprotic phosphoric acid system. The loss of a positive
charge results in the charge moving from -1 to -2. Recognizing conjugate pairs is a key skill
in predicting the direction of acid-base reactions.
4. A solution has a pOH of 4.75. What is the pH of this solution at 25 degrees Celsius?
A. 9.25
B. 7.00
C. 4.75
D. 10.25
Correct Answer: A
Expert Explanation: At the standard temperature of 25 degrees Celsius, the sum of pH and
pOH is always 14.00. To find the pH, you subtract the given pOH from 14.00. Therefore,
14.00 minus 4.75 equals 9.25. This relationship stems from the ion product constant of
water (Kw), which is 1.0 x 10^-14. Solutions with a pH higher than 7, such as this one, are
considered basic.
5. According to Le Châtelier’s Principle, how will the equilibrium 2SO2(g) + O2(g) ⇌ 2SO3(g)
shift if the pressure is increased?
A. Shift to the left toward the reactants.
B. No shift will occur.
C. Shift to the right toward the products.
D. The equilibrium constant will change.
Correct Answer: C
Expert Explanation: Increasing the pressure of a gaseous system at equilibrium shifts the
reaction toward the side with fewer moles of gas. In this reaction, there are 3 moles of gas
on the left and 2 moles on the right. Therefore, the system moves to the right to reduce the
total number of particles and alleviate the pressure. This shift helps the system regain
equilibrium by counteracting the external stress. Understanding these shifts is vital for
industrial chemical production processes.
6. Which pair of substances would be most suitable for preparing a buffer solution with a pH
of approximately 4.7?
A. NH3 and NH4Cl (pKa of NH4+ = 9.25)
, B. NaOH and NaCl
C. HCl and NaCl
D. CH3COOH and NaCH3COO (pKa = 4.74)
Correct Answer: D
Expert Explanation: A buffer is most effective when the desired pH is close to the pKa of
the weak acid component. Acetic acid (CH3COOH) has a pKa of 4.74, making it an excellent
choice for a buffer near pH 4.7. Strong acids and strong bases like HCl or NaOH cannot form
buffers because they do not exist in equilibrium with a conjugate pair. The ratio of acid to
conjugate base is typically 1:1 at the pKa value. This selection ensures maximum buffering
capacity against both acid and base additions.
7. In the reaction NH3 + H2O ⇌ NH4+ + OH-, which species acts as the Brønsted-Lowry acid?
A. NH3
B. NH4+
C. H2O
D. OH-
Correct Answer: C
Expert Explanation: A Brønsted-Lowry acid is defined as a proton donor. In this specific
forward reaction, water (H2O) donates a hydrogen ion to ammonia (NH3). This results in
the formation of the hydroxide ion and the ammonium ion. Ammonia acts as the base
because it accepts the proton from the water molecule. Therefore, water serves as the acid
in this specific equilibrium context.
8. What happens to the pH of a buffer solution when a small amount of strong acid is added?
A. The pH decreases slightly.
B. The pH decreases significantly.
C. The pH stays exactly the same.
D. The pH increases significantly.
Correct Answer: A
Expert Explanation: Buffer solutions are designed to resist large changes in pH upon the
addition of acids or bases. When a strong acid is added, the conjugate base in the buffer
reacts with the added H+ ions. This conversion minimizes the increase in free hydronium
ion concentration in the solution. Consequently, while the pH does drop slightly, it does not
drop drastically as it would in pure water. This stability is crucial for biological systems and
chemical manufacturing.
Updated and Latest Questions and Correct
Answers with Rationale - Portage Learning
1. According to the Brønsted-Lowry theory, what is the definition of a base?
A. A substance that produces OH- ions in aqueous solution.
B. A substance that acts as a proton (H+) acceptor.
C. A substance that acts as an electron pair donor.
D. A substance that produces H+ ions in aqueous solution.
Correct Answer: B
Expert Explanation: The Brønsted-Lowry theory specifically defines a base as any species
capable of accepting a proton. This definition focuses on the transfer of H+ ions rather than
just the production of hydroxide ions in water. For example, ammonia acts as a base by
accepting a proton to form ammonium. Unlike the Arrhenius definition, this allows for base
behavior in non-aqueous environments. This concept is fundamental to understanding
acid-base conjugate pairs in equilibrium.
2. What is the pH of a 0.025 M solution of Hydrochloric Acid (HCl)?
A. 12.40
B. 1.40
C. 2.50
D. 1.60
Correct Answer: D
Expert Explanation: Hydrochloric acid is a strong acid that dissociates completely in
water to provide H+ ions. The pH is calculated using the formula pH = -log[H+]. Inserting
0.025 M into the equation gives -log(0.025), which equals approximately 1.60. Since HCl is
monoprotic, the concentration of the acid is equal to the concentration of the hydronium
ions. Understanding this logarithmic scale is essential for quantifying acidity in chemical
reactions.
3. Which of the following represents the conjugate base of the dihydrogen phosphate ion
(H2PO4-)?
A. HPO4^2-
B. H3PO4
C. PO4^3-
,D. OH-
Correct Answer: A
Expert Explanation: A conjugate base is formed when an acid loses a single proton (H+).
For H2PO4-, removing one H+ results in the formation of HPO4^2-. This relationship is part
of a series of equilibria for the polyprotic phosphoric acid system. The loss of a positive
charge results in the charge moving from -1 to -2. Recognizing conjugate pairs is a key skill
in predicting the direction of acid-base reactions.
4. A solution has a pOH of 4.75. What is the pH of this solution at 25 degrees Celsius?
A. 9.25
B. 7.00
C. 4.75
D. 10.25
Correct Answer: A
Expert Explanation: At the standard temperature of 25 degrees Celsius, the sum of pH and
pOH is always 14.00. To find the pH, you subtract the given pOH from 14.00. Therefore,
14.00 minus 4.75 equals 9.25. This relationship stems from the ion product constant of
water (Kw), which is 1.0 x 10^-14. Solutions with a pH higher than 7, such as this one, are
considered basic.
5. According to Le Châtelier’s Principle, how will the equilibrium 2SO2(g) + O2(g) ⇌ 2SO3(g)
shift if the pressure is increased?
A. Shift to the left toward the reactants.
B. No shift will occur.
C. Shift to the right toward the products.
D. The equilibrium constant will change.
Correct Answer: C
Expert Explanation: Increasing the pressure of a gaseous system at equilibrium shifts the
reaction toward the side with fewer moles of gas. In this reaction, there are 3 moles of gas
on the left and 2 moles on the right. Therefore, the system moves to the right to reduce the
total number of particles and alleviate the pressure. This shift helps the system regain
equilibrium by counteracting the external stress. Understanding these shifts is vital for
industrial chemical production processes.
6. Which pair of substances would be most suitable for preparing a buffer solution with a pH
of approximately 4.7?
A. NH3 and NH4Cl (pKa of NH4+ = 9.25)
, B. NaOH and NaCl
C. HCl and NaCl
D. CH3COOH and NaCH3COO (pKa = 4.74)
Correct Answer: D
Expert Explanation: A buffer is most effective when the desired pH is close to the pKa of
the weak acid component. Acetic acid (CH3COOH) has a pKa of 4.74, making it an excellent
choice for a buffer near pH 4.7. Strong acids and strong bases like HCl or NaOH cannot form
buffers because they do not exist in equilibrium with a conjugate pair. The ratio of acid to
conjugate base is typically 1:1 at the pKa value. This selection ensures maximum buffering
capacity against both acid and base additions.
7. In the reaction NH3 + H2O ⇌ NH4+ + OH-, which species acts as the Brønsted-Lowry acid?
A. NH3
B. NH4+
C. H2O
D. OH-
Correct Answer: C
Expert Explanation: A Brønsted-Lowry acid is defined as a proton donor. In this specific
forward reaction, water (H2O) donates a hydrogen ion to ammonia (NH3). This results in
the formation of the hydroxide ion and the ammonium ion. Ammonia acts as the base
because it accepts the proton from the water molecule. Therefore, water serves as the acid
in this specific equilibrium context.
8. What happens to the pH of a buffer solution when a small amount of strong acid is added?
A. The pH decreases slightly.
B. The pH decreases significantly.
C. The pH stays exactly the same.
D. The pH increases significantly.
Correct Answer: A
Expert Explanation: Buffer solutions are designed to resist large changes in pH upon the
addition of acids or bases. When a strong acid is added, the conjugate base in the buffer
reacts with the added H+ ions. This conversion minimizes the increase in free hydronium
ion concentration in the solution. Consequently, while the pH does drop slightly, it does not
drop drastically as it would in pure water. This stability is crucial for biological systems and
chemical manufacturing.
