h？ CO？ It is carbonic acid, weak acid, the first-order ionization constant Ka=4.3× 10, and its solubility is low. When the concentration of aqueous solution is slightly higher, carbon dioxide gas will evaporate. Its yoke group is bicarbonate ion HCO, such as potassium bicarbonate and sodium bicarbonate. The solutions are all alkaline.

h？ c？ o？ Oxalic acid, the scientific name of oxalic acid, is almost the strongest among organic acids. The first-order ionization constant Ka = 5.9×10; Its * * * yoke group is oxalate ion HC? O, such as sodium oxalate, the solution is alkaline.

h？ PO is dihydrogen phosphate ion. Because phosphoric acid is a ternary acid, dihydrogen phosphate ions can still ionize hydrogen ions, and its ionization constant (secondary ionization constant of phosphoric acid) Ka=6.23× 10, which is weaker than carbonic acid, but the solubility of most dihydrogen phosphates is much higher than carbonic acid, and they are acidic with salt solutions composed of alkali metals. Its yoke base is dihydrogen phosphate HPO? All alkali metal salts are alkaline.

HAc is the abbreviation of acetic acid (acetic acid), and the chemical formula is ch? COOH, weak acid, its ionization constant Ka= 1.76× 10, and it is miscible with water in any proportion; Its yoke base is acetate ion Ac? (CH？ Chief operating officer? ), the acetate of alkali metal is alkaline.

HF is hydrofluoric acid, a weak acid with ionization constant Ka=3.53× 10, which is miscible with water in any proportion. Its yoke base is fluoride ion F? Alkali metal fluorides are all alkaline.

NH is ammonium ion with ionization constant Ka=5.64× 10? Most of its salts are soluble in water, and the strong acid ammonium salt is acidic; Its yoke base is ammonia NH? h？ O, weak base, ionization constant Kb= 1.774× 10, ammonia has high solubility, but it will volatilize ammonia.

If we want to form a buffer pair, we need to consider its acidity and solubility.

(1), PH=2.0, hydrogen ion concentration 10mol/L, h? c？ o？ , HF meets the requirements, HAc concentration is high, but it is feasible.

h？ CO？ /HCO concentration ≈( 10)? ⊙ (4.3×10)+10 ≈ 233 mol/L, which is definitely beyond the solubility range and is not feasible;

h？ c？ o？ /HC？ O concentration ≈( 10)? (5.9×10)+10 ≈1.17×10 mol/l, which is within the solubility range and meets the requirements.

h？ Po /HPO? Concentration ≈( 10)? ⊙ (6.23×10)+10 ≈1605 mol/l, which is definitely out of range and not feasible.

HAc/Ac？ Concentration ≈( 10)? ⊙ (1.76×10)+10 ≈ 5.69 mol/L, which is equivalent to about 341.5 g/L. The concentration is high, but it is feasible.

HF/F？ Concentration ≈( 10)? ⊙ (3.53×10)+10 ≈ 0.293 mol/L, which is within the solubility range and meets the requirements.

NH/NH？ Concentration ≈( 10)? ÷(5.64× 10? )+10≈ 177304mol/L, which is definitely out of range and not feasible.

(2), PH=7.0, hydrogen ion concentration is 10mol/L, which is neutral and can be used.

All the above six substances are acidic (strong acid ammonium salts are also acidic), and their yokes can be alkaline, neutralizing each other, and all can reach neutrality at PH=7.0, so they are all feasible.

(3), PH=9.0, hydroxide ion concentration 10mol/L, only oxalic acid is not feasible, others can be used.

h？ CO？ /HCO concentration ≈( 10)? ÷ 10? × (4.3×10)+10 = 0.00431mol/l, which is within the solubility range of alkali metal bicarbonate and meets the requirements.

h？ c？ o？ /HC？ O concentration ≈( 10)? ÷ 10? × (5.9×10)+10 ≈ 590 mol/L, which is definitely beyond the concentration range and is not feasible.

h？ Po /HPO? Concentration ≈( 10)? ÷ 10? × (6.23×10)+10 ≈ 6.33×10 mol/L, which is within the solubility range of dibasic phosphate and meets the requirements.

HAc/Ac？ Concentration ≈( 10)? ÷ 10? × (1.76×10)+10 ≈ 0.176 mol/l, which is within the solubility range of acetate and meets the requirements.

HF/F？ Concentration ≈( 10)? ÷ 10? × (3.53x10)+10 ≈ 3.53mol/l, which is equivalent to 148 g/L sodium fluoride. It is within the solubility range and meets the requirements.

NH/NH？ Concentration ≈( 10)? ÷ 10? ×(5.64× 10? )+10 ≈1.564×10 mol/l, which is within the solubility range of ammonia gas and meets the requirements.

Although the above conclusion is feasible in theory, in practice, due to H? CO？ The solubility is low, and the PH balance of the prepared buffer solution is easily broken, so the feasibility is poor.