الكيمياء العضوية عمليLab4.pdf

National University of Science and Technology College of Science First year level Organic Chemistry Labs Supervisor: Ass.Lec.Bassam Wathgiq Abdulateef Ass.Lec. Sarah Salah Jalil Lab – 4 – Silver nitrate titrations by...

National University of Science and Technology College of Science First year level Organic Chemistry Labs Supervisor: Ass.Lec.Bassam Wathgiq Abdulateef Ass.Lec. Sarah Salah Jalil Lab – 4 – Silver nitrate titrations by (Mohr method) 1. Aim: Calculate NCl- or NBr- 2. Introduction: NaCl (soln.) + AgNO3(titrant) → AgCl(white ppt.) + NaNO3 At the end point: AgNO3(titrant) + K2CrO4 → KNO3 + Ag2CrO4(red ppt.) In the precipitation of chloride by silver ion, chromate ion (CrO 42−) is used as an indicator in the formation of Ag2CrO4, a reddish-brown precipitate formed when excess Ag+ is present. Ag+ + Cl− AgCl(s) titrant analyte white precipitate Mohr indicator reaction (end point), 2Ag+ + CrO42− Ag2CrO4(s) titrant indicator red precipitate The concentration of titrant rises sharply near the equivalence point, and the solubility of Ag2CrO4 is exceeded. The titrations are performed only in neutral or slightly basic medium to prevent silver hydroxide formation (at pH > 10). 2Ag+ + 2OH− 2AgOH(s) Ag2O(s) + H2O Precipitate Or the formation of chromic acid at pH < 7 CrO42− + H3O+--------- HCrO4− + H2O 1  2CrO42− + 2H3O+--------Cr2 O7 2− + H2O [CrO42−] become lower, more Ag+ to be added to reach end point, which cause error. 3. Chemicals: 1- AgNO3 (0.1N). 2- NaCl (unknown). 1- K2CrO4 (Indicator) 4. Apparatus: 1-Conical flask. 2-Burette. 3- Pipitte. 4- Beaker. 5- Stand & clamp. 6- Dropper. 5. Procedure: 1- Transfer 10 ml of NaCl solution in C.F. 2- Add 3 drops of chromate indicator pH(6.5-9) 3- Titrate against 0.1N AgNO3 (white ppt. is formed) 4- At end point the excess of AgNO3 will react with Ind. To form red ppt. 6. Calculations: (N x V)NaCl = (N x V)AgNO3 Burette reading Difference Exp. Initial Final 1 2 3 2