Ionization equilibria, solubility products, and buffer action in aqueous solutions are essential for understanding biological systems and environmental chemistry.
Basic Knowledge
① Ion product of water: Kw=[H+][OH-]=1.0×10-14(25℃)
② pH=-log[H+]. Neutral pH=7
③ Acid ionization constant Ka: HA⇌H++A-, Ka=[H+][A-]/[HA]. pKa=-log Ka
④ Buffer solution: weak acid and its salt or weak base and its salt — the pH changes only slightly when a small amount of acid or base is added. Henderson–Hasselbalch equation pH = pKa + log([A-]/[HA])
⑤ Solubility product Ksp: AgCl(s)⇌Ag++Cl-, Ksp=[Ag+][Cl-]=1.8×10-10
Kw（Ion product of water=10-14(25℃)）
Ka, pKa（Indicators of acid strength）
Henderson equation（pH = pKa + log([A-]/[HA])）
Buffer solution（Weak acid+salt or weak base+salt）
Solubility product Ksp（Solubility equilibrium of a sparingly soluble salt）
Common-ion effect（Solubility decreases when a common ion is added）
Further Exploration (Principles and Applications)
Blood has a pH of approximately 7.4, and the CO2/HCO3- buffer system plays a central role: CO2+H2O⇌H2CO3⇌H++HCO3-. When the pH falls below 7.35, the condition is acidosis; when it exceeds 7.45, it is alkalosis, both of which are life-threatening.
Using solubility products, the selective precipitation of sulfides in qualitative analysis(Ag2S and CuS precipitate even in acidic aqueous solutions, while ZnS and MnS precipitate under basic conditions) and the use of solubility differences among AgCl/AgBr/AgI in photographic development can be treated quantitatively.
- Kw=10-14(25℃)
- pH+pOH=14
- Buffer solution=weak acid+conjugate salt
- Henderson: pH=pKa+log([A-]/[HA])
- Buffer capacity is greatest at pKa±1
- Ksp=expression of the solubility of a sparingly soluble salt
- Solubility decreases due to the common-ion effect
Points to Note (Common Confusions and Frequent Mistakes)
① A buffer solution consists of a weak acid and its conjugate base(or the reverse). A strong acid and strong base do not form a buffer. ② Kw increases as temperature rises (approximately 2.9×10-14 at 40℃), so neutral pH also changes. ③ Ksp is not dimensionless and has units. ④ Common-ion effect: adding NaCl to AgCl decreases the solubility of AgCl.
Practice
- What is [H+] in an aqueous solution with pH=3? Also find [OH-].
- Find the pH of a buffer solution containing 0.10 mol/L CH3COOH(Ka=1.8×10-5) and 0.10 mol/L CH3COONa.
- When the solubility product of AgCl is Ksp=1.8×10-10, find the solubility of AgCl in pure water [mol/L].