Experiment 3: Analysis of a Mixture of Carbonate and Bicarbonate Andrew Mullenax Objective: The objective of this lab is to determine the amount of two different carbonate species via direct and indirect titration methods. Reactions: π»πΆπ3− + π» + → π»2 πΆπ3 πΆπ32− + 2π» + → π»2 πΆπ3 π»πΆπ3− + ππ» − → πΆπ3− + π»2 π π΅π2+ + πΆπ32− → π΅ππΆπ3 π΅π2+ + ππ» − → π΅π(ππ»)2 Procedure: ο· ο· ο· ο· ο· ο· ο· ο· ο· ο· ο· ο· Obtain an unknown which contains a mixture of the carbonates of interest and store it in the dessicator Prepare approximately 0.1M HCl and approximately 0.1M NaOH using primary standard grade KHP Accurately weigh enough KHP to require at least 25mL of titrant Use this to standardize your NaOH Repeat until 3 good trials are obtained Repeat using NaOH to standardize your acid Weigh 2.0-2.5g of unknown into a 250mL Erlenmeyer flask. Dilute to the mark with freshly boiled and triply distilled water Pipet a 25.00mL aliquot of unknown into a 250mL Erlenmeyer flask and titrate with standardized HCl using bromocresol green to determine the end point Repeat until 3 good trials are obtained Pipet a 25.00mL aliquot of unknown and a 50.00mL aliquot of standard NaOH into a 250mL Erlenmeyer flask Add 10mL of 10wt% BaCl2, swirl to precipitate all BaCO3 then immediately titrate with standard HCl using phenolphthalein indicator. Repeat until 3 good trials are obtained Data: Volume of 6M HCl used = 16.6mL Mass NaOH used = 2.00g Mass KHP used = 0.51g Bold = Bad trial Standardization of NaOH Trial 1 Mass KHP(g) .5099 Trial 2 .5091 Trial 3 .5098 Trial 4 .5105 Average 0.5098 Volume NaOH added(mL) Concentration of NaOH(M) 27.50 26.74 26.57 26.60 26.85 .0908 .0932 .0940 .0940 0.0937 Standardization of HCl Volume HCl Added(mL) Volume NaOH(mL) Concentration of HCl(M) Trial 1 Trial 2 Trial 3 Average 25.13 25.11 25.13 25.12 25.0 25.0 25.0 25.0 .0932 .0933 .0932 0.0932 Mass Unknown = 2.0368g Titration of Unknown(using bromocresol green) Trial 2 Trial 1 Volume HCl 28.55 29.50 added(mL) Moles of Carbonate .00266 .00275 and Bicarbonate Titration of Unknown(using phenolphthalein) Trial 2 Trial 1 Volume HCl 29.33 29.80 added(mL) Moles of NaOH in .00273 .00278 excess Trial 3 Trial 4 Average 28.60 28.66 28.83 .00267 .00267 0.00269 Trial 3 Trial 4 Average 29.50 29.49 29.53 .00275 .00275 0.00275 Total Moles of NaOH = .004685 Results Moles of Bicarbonate Trial 1 Trial 2 Trial 3 Trial 4 Average Standard Deviation .00191 .00196 .00194 .00194 0.00194 .00002 Moles of Carbonate wt% Bicarbonate wt% Carbonate .00084 .00070 .00073 .00073 0.00075 .00006 5.72 5.87 5.81 5.81 5.8 .06185 2.47 2.06 2.15 2.15 2.21 .18007 Calculations: Moles of KHP: πππ π πΎπ»π π₯ . 5099π π₯ 1 πππ 204.2212π 1 πππ = .00250 πππππ πΎπ»π 204.2212π Concentration of NaOH: πππππ πΎπ»π 1000ππΏ π₯ ππππ’ππ ππππ» πππππ 1πΏ . 00250 πππππ 1000ππΏ π₯ = .0908π 27.50 ππΏ 1πΏ Concentration of HCl: π£πππ’ππ ππππ» π₯ πππππππ‘πππ‘πππ ππππ» π£πππ’ππ π»πΆπ πππππ 25.0ππΏ π₯ .0937π = .0932π 25.13ππΏ Moles of Carbonate and Bicarbonate: ππππ’ππ π»πΆπ πππππ π₯ πππππππ‘π¦ ππ π»πΆπ 1000 29.50ππΏ π₯ .0932π = .00275 πππππ 1000 Moles of excess NaOH: ππππ’ππ π»πΆπ πππππ π₯ 29.80ππΏ π₯ 1πΏ . 0932πππ π₯ 1000ππΏ 1πΏ 1πΏ . 0932πππ π₯ = .00278 πππππ 1000ππΏ 1πΏ Total Moles of NaOH: ππππππππ π£πππ’ππ ππππ» π₯ πππππππ‘π¦ ππππ» . 05πΏ π₯ .0937π = .004685 πππππ ππππ» Moles of NaOH reacted: πππ‘ππ πππππ ππππ» − πππππ ππ ππ₯πππ π ππππ» . 004685 πππππ − .00278 πππππ = .00191 πππππ ππππ» Moles Carbonate: πππππ πΆπππππππ‘π πππ ππππππππππ‘π − πππππ ππππ» πππππ‘ππ . 00275πππππ − .00191πππππ = .00084πππππ πΆπ32− Wt% bicarbonate: πππππ π»πΆπ3− π₯ 61π/πππ π₯ . 00191 πππππ π₯ 1 π₯ 100 πππ π π’πππππ€π 61π 1 π₯ π₯ 100 = 5.72% 1 πππ 2.0368π Wt% carbonate: πππππ πΆπ32− π₯ . 00084 πππππ π₯ 60π 1 π₯ π₯ 100 1 πππ πππ π π’πππππ€π 60π 1 π₯ π₯ 100 = 2.47% 1 πππ 2.0368π Conclusions: The purpose of this lab was to determine the relative weight percentage of carbonate and bicarbonate in an unknown sample. The average weight percentages of bicarbonate and carbonate were determined to be 5.80% and 2.21% respectively. Post-lab Questions: A. A primary standard is a pure/stable reagent that can be used directly after weighing. B. A secondary standard is a standard that has been standardized using a primary standard. C. An indirect titration is one in which an excess of standard reagent is added to the analyte and is titrated endpoint. D. The titrant is a reagent of known concentration used to titrate the unknown unknown.