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11 7. Calculate the pHof the solution, at which the formation of Cu (OH) 2  crystals begins, if the concentration of Cu  2+ ions in the solution is 0.01 mol/l? K S (Cu (OH) 2 ) = 8.3 ∙ 10 –20. Answer : pH> 5.46. 8. 20 ml of a 0.05M solution of Cd (NO 3 ) 2  were mixed with 30 ml of a 0.02M solution of K 2 S? Is there a precipitate of CdS? K S (CdS) = 1.6∙10 –28. Laboratory work Experiment 1. Precipitation conditions In two tubes take 1 ml of 0.01 mol/l Pb (NO 3 ) 2. Add 1 ml of 0.01 mol/l NaCl solution to the first tube, and 1 ml of 1 mol/l NaCl solution into the second tube. What are you watching? Based on the calculated data, explain why in one case a precipitate falls out, and in another – no? K S (PbCl 2 ) = 1.7∙10 –5. Write the reaction’sequation. Experiment 2. Determination of the degree of completeness of precipitation The precipitate obtained in the first experiment is filtered out. Add 1 drop of NaCl solution 1 mol/l to the filtrate to determine the completeness of the precipitation of lead (II) chloride, then add 5 drops of Na 2 SO 4. Explain the formation of a white precipitate of PbSO 4. Write the reaction’s equation. What other ions can precipitate lead ions after the precipitation of lead chloride? Write a short explanation. Experiment 3. Precipitation and dissolution conditions Pour 1 ml of MgCl 2  solutionin a volumetric test tube, then add 1 ml of NaOH solution. The obtained precipitate is divided into two test tubes. Add 1 ml of HCl solution to the first test tube, in the other – 1 ml of NH 4 Cl solution. What are you watching? Write the equations for the dissolution of precipitation. Pour 1 ml of BaCl 2  solutionin a volumetric test tube, then add 1 ml of Na 2 SO 4  solution. The obtained precipitate is divided into two test tubes. Add 1 ml of HCl solution to the first test tube, in the other – 1 ml of NH 4 Cl solution. What are you watching? Give an explanation. LESSON 6. BASICS OF CHEMICAL THERMODYNAMICS.  THERMOCHEMISTRY. THE LAWOFHESS. ENTROPY. GIBBS ENERGY Required base level. Thermal effect of chemical reaction. Exothermic and endothermic reactions. Types of thermodynamic processes: isothermal, isobaric, isochoric.