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Precipitation equilibria and pH

Question: what happens when we add 1 mol of Hg(CH 3 COO) 2 to 1 L of water?. Precipitation equilibria and pH. Hg(CH 3 COO) 2 ( s ) → Hg 2+ ( aq ) + 2 CH 3 COO – ( aq ). CH 3 COO – ( aq ) + H 2 O → CH 3 COOH ( aq ) + OH – ( aq ). pK a = 4.74 ⇒ pK b = 9.26. K b = 5.50 ×10 –10.

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Precipitation equilibria and pH

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  1. Question: what happens when we add 1 mol of Hg(CH3COO)2 to 1 L of water? Precipitation equilibria and pH • Hg(CH3COO)2 (s) → Hg2+ (aq) + 2 CH3COO– (aq) • CH3COO– (aq) + H2O → CH3COOH (aq) + OH– (aq) pKa = 4.74 ⇒ pKb = 9.26 Kb= 5.50 ×10–10 x = [OH–] = 3.3 ×10–5 x2/(2 – x) = 5.50 × 10–10 • Hg(OH)2 (s) ⇌ Hg2+ (aq) + 2 OH– (aq) • But Hg(OH)2 has a small solubility product! Qsp>> Ksp Ksp= 3.0 ×10–26 Qsp= 1 × (3.3 ×10–5) = 1.1 ×10–9 • So if we try to dissolve mercury (II) acetate in ordinary water, Hg(OH)2 will precipitate! CHEM 114 CHEM 114 Fundamental Chemistry • Hg(OH)2 decomposes to give H2O + HgO, which is orange.

  2. The same will happen with any salt of a weak acid if the hydroxide is very insoluble. Why you can’t buy real aluminum acetate... CHEM 114 CHEM 114 Fundamental Chemistry

  3. Qualitative inorganic analysis ‘Classical’ scheme breaks up group 3 into two groups (3 and 4) CHEM 114 Fundamental Chemistry

  4. Group I: the insoluble chlorides. • Add 1 - 2 M HCl. AgCl, Hg2Cl2 and PbCl2 precipitate. Qualitative inorganic analysis • If stronger HCl is used, Pb may re-dissolve as the complex cation PbCl4– • Distinguishing between them: PbCl2 is quite soluble in hot water. • AgCl is soluble in ammonia — the chloride of the complex cation Ag(NH3)2+ is soluble. • Hg2Cl2 precipitate turns black in the presence of ammonia. • PbCl2 is not that insoluble; at 1 M Cl–, 16 µM Pb2+ feeds through to group 2 CHEM 114 Fundamental Chemistry • If results are ambiguous, test for Pb2+ with chromate • Group I ppt. • Test for Pb • Test for Hg

  5. Group 2: the acid-insoluble sulfides. • Add 0.25 M H2S or 0.4 M thioacetamide. Cd2+, Bi3+, Cu2+, As3+,Sb3+, Sn2+, Sn4+ and Hg2+ precipitate as the canonical sulfides. Pb2+ may leak through from group 1. Qualitative inorganic analysis pKa ~ 7 H2S (aq.) ⇌ HS–(aq.) + H+(aq.) pKa ~ 19 HS–(aq.) ⇌ S2–(aq.) + H+(aq.) Because there is very little S2– at acid pH, only the most insoluble sulfides precipitate Reddish brown or black precipitate: Bi3+, Cu2+, Hg2+ or Pb2+ Yellow precipitate: Cd2+ or Sn4+. SnS2 dissolves in aqueous NaOH Red-orange precipitate: Sb3+ Brown precipitate: Sn2+ HgS insoluble in HNO3. Soluble material treated with H2SO4 (PbSO4 precipitates) CHEM 114 Fundamental Chemistry CuS, CdS soluble in aqueous ammonia (Cu blue, Cd colorless)

  6. Group 3: the ammonia-insoluble sulfides. • Add H2S and NH3. Fe2+, Fe3+, Al3+, and Cr3+ precipitate as the sulfides at pH 8 - 9 with or without ammonia. Zn2+, Ni2+, Co2+, and Mn2+ precipitate if NH3 is present Qualitative inorganic analysis Reddish brown precipitate: Fe3+ Gelatinous white precipitate: Al3+ Green precipitate: Fe2+ or Cr3+. Fe2+ dissolves in excess NaOH Second group: white precipitate: Zn2+ Colored precipitate (pink - brown): Mn2+ Black precipitate (pink - brown): Ni2+ or Co2+, Ni2+ if the solution was green CHEM 114 Fundamental Chemistry

  7. Group 4: the insoluble carbonates. • Add 0.2 M (NH4)2CO3. Mg2+, Ca2+, Sr2+, Ba2+ precipitate. Qualitative inorganic analysis All precipitates are white. Distinguish via flame test. • Ba2+ flame apple green. • Sr2+ flame deep red. • Ca2+ flame brick red. What’s left? Li+, Na+, K+. Distinguish via flame test. CHEM 114 Fundamental Chemistry

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