U8.4 · Acid-Base Reactions and BuffersPractice · 酸碱反应与缓冲溶液
Lessons covered:U8.4 Acid-Base Reactions and Buffers
- 1Multiple choice
What is the net ionic equation for the reaction of HCN solution with NaOH solution?
ExplanationHCN is a weak acid, present mostly as HCN molecules, so it is written as a molecule; NaOH is a strong base and Na⁺ is a spectator. HCN hands its H⁺ straight to OH⁻. H⁺ + OH⁻ → H₂O applies only to a strong acid with a strong base, the third choice is the molecular equation, and the fourth is CN⁻ reacting with water.
- 2Numeric
40.0 mL of 0.10 M HNO₂ (pKa = 3.34) is mixed with 20.0 mL of 0.10 M NaOH. What is the pH of the mixture?
ExplanationHNO₂ is 4.0 mmol and OH⁻ is 2.0 mmol. Afterward 2.0 mmol HNO₂ remains and 2.0 mmol NO₂⁻ has formed: equal amounts, a buffer, so pH = pKa + log 1 = 3.34.
- 3Numeric
20.0 mL of 0.10 M HCN (Ka = 4.9 × 10−10) is mixed with 20.0 mL of 0.10 M NaOH. What is the pH of the mixture at 25 °C?
ExplanationThey react exactly, giving 2.0 mmol of CN⁻ in 40.0 mL, so [CN⁻] = 0.050 M. Kb = Kw ÷ Ka = 2.0 × 10−5, [OH⁻] ≈ √(2.0 × 10−5 × 0.050) = 1.0 × 10−3 M, pOH = 3.00, pH = 11.00. The weaker the acid HCN, the stronger the base CN⁻.
- 4Multiple choice
Which of these mixtures gives a buffer solution?
ExplanationWork out the reaction first: 1.0 mmol HNO₂ + 0.5 mmol OH⁻ leaves 0.5 mmol HNO₂ and makes 0.5 mmol NO₂⁻, a weak acid and its conjugate base together, so it’s a buffer. Equal HNO₂ and KOH react completely, leaving only NO₂⁻; NH₃ with excess HCl leaves strong acid; and Cl⁻ in HCl/KCl is essentially not basic.
- 5Select all
At 25 °C, which of these 0.10 M salt solutions are acidic? Select all that apply.
More than one option may be correct. Choose all that apply, then press Submit.
ExplanationNH₄⁺ and CH₃NH₃⁺ are conjugate acids of weak bases, so they donate H⁺ and make the solution acidic; NO₃⁻ and Cl⁻ are conjugate bases of strong acids and don’t affect the pH. CN⁻ and CO₃²⁻ are conjugate bases of weak acids, so those solutions are basic, and NaCl is neutral.
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