Conceptual
Why can’t a mixture of HCl and NaCl act as a buffer?
A.
Because HCl is a strong acid and completely dissociates, leaving no weak acid to react with added bases.
B.
Because NaCl reacts with HCl to form water, eliminating the buffer components.
C.
Because HCl and NaCl are both salts, so they cannot affect pH.
D.
Because strong acids cannot exist in aqueous solutions.
Write the net ionic equation for the formation of AgCl when AgNO₃ reacts with NaCl, and explain why AgCl is insoluble.
A.
Ag⁺(aq) + Cl⁻(aq) → AgCl(s)
B.
AgNO₃(aq) + NaCl(aq) → AgCl(aq) + NaNO₃(s)
C.
Ag⁺(aq) + NO₃⁻(aq) → AgNO₃(s)
D.
Na⁺(aq) + Cl⁻(aq) → NaCl(s)
Explain why a mixture of CH₃COOH and CH₃COONa can resist pH changes when small amounts of acid or base are added.
A.
Because CH₃COOH and CH₃COONa react with each other to form a strong acid that neutralizes any added acid or base.
B.
Because the mixture contains both a weak acid (CH₃COOH) and its conjugate base (CH₃COO⁻), which can neutralize added acids and bases, maintaining the pH.
C.
Because CH₃COONa completely neutralizes the CH₃COOH, producing a neutral solution that cannot change pH.
D.
Because both substances are strong electrolytes that prevent any ions from reacting in solution.
A buffer solution contains equal concentrations of NH₄⁺ and NH₃. If NaOH is added, describe what happens to the ratio of NH₄⁺ to NH₃.
A.
The ratio increases because NH₄⁺ reacts with OH⁻ to form more NH₃.
B.
The ratio decreases because NH₃ reacts with OH⁻ to form NH₄⁺.
C.
The ratio stays the same because NaOH does not react with the buffer components.
D.
The ratio becomes zero because all NH₄⁺ is converted to NaOH.
Predict whether a precipitate will form when solutions of BaCl₂ and Na₂SO₄ are combined. Explain using solubility rules.
A.
Yes, BaSO₄ will precipitate because barium sulfate is insoluble, while sodium chloride remains dissolved.
B.
No, all products are soluble, so no precipitate forms.
C.
Yes, NaCl will precipitate because sodium salts are generally insoluble.
D.
No, BaCl₂ reacts with Na₂SO₄ to form a gas instead of a precipitate.
If a solution has a pH close to the pKa of the acid in its buffer, why is this condition ideal for maintaining pH stability?
A.
Because the concentrations of the weak acid and its conjugate base are approximately equal, allowing the buffer to neutralize added acids or bases effectively.
B.
Because the solution becomes neutral at this point, so no pH changes occur.
C.
Because the strong acid completely dissociates, maximizing the buffer capacity.
D.
Because water molecules prevent any ionization of the acid, stabilizing pH.