Química Ambiental e Precipitação

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18 Chemistry of the Environment Solutions to Exercises 7.0 X 10⁻⁴ mol 1 mol Ca(OH)₂ X X 1.200 X 10³ L = 0.42 mol Ca(OH)₂ L 2 mol 1.200 X 10³ L 5.0 X 10⁻⁴ L mol = 0.60 mol (aq) total 0.42 mol Ca(OH)₂ removes 0.42 mol of the 0.60 mol (aq) in the sample. This leaves 0.18 mol (aq) to be removed by + Na₂CO₃(aq) 0.18 mol of is needed to remove the remaining Ca²⁺(aq). 18.52 Ca(OH)₂ is added to remove Ca²⁺ as CaCO₃(s), and removes the remaining Ca²⁺. + (aq) + + 2CaCO₃(s) + One mole Ca(OH)₂ is needed for each 2 moles of (aq) present. 5.0 10⁷ H2O X 1.7 X 10⁻³ 1 2 mol mol Ca(OH)₂ 1 74 mol Ca(OH)₂ Ca(OH)₂ = 3.1 X 10⁶ g Ca(OH)₂ Half of the native precipitates the added Ca²⁺ so this operation reduces the Ca²⁺ concentration from 5.7 M to (5.7 10⁻³ - 8.5 10⁻⁴) M = 4.85 10⁻³ = 4.9 10⁻³ M. Next we must add sufficient to further reduce [Ca²⁺] to 1.1 X M (20% of the original [Ca²⁺]). We thus need to reduce [Ca²⁺] by (4.85 X 10⁻³ - 1.1 M = 3.75 = 3.8 M Ca (aq) + CaCO₃(s). 5.0 10⁷ L 3.75 X 10⁻³ mol X 1 mol 1 mol 1 mol = 2.0 10⁷ g 18.53 + 4OH⁻(aq) + (aq) is a spectator, so the net ionic equation is (aq) + (aq) + + + In this reaction, Fe³⁺ acts as a Lewis acid, and acts as a Lewis base. 18.54 is a typical coagulant in municipal water purification. It reacts with OH- in a slightly basic solution to form a gelatinous precipitate that occludes very small particles and bacteria. The precipitate settles slowly and is removed by sand filtration. Properties of (SO₄)₃ and other useful coagulants are: They react with low concentrations of That is, of the hydroxide precipitate is very small. The capacity to form a hydroxide precipitates means that no extra salts must be added to form the precipitate. Also, the [OH⁻] can be easily adjusted by Ca(OH)₂ and other reagents that are part of the purification process. 561