PHARCHM 3 LECTURE nd 2 Term AY 2016-2017
PROBLEM SET ON NEUTRALIZATION TITRATION
1. A 0.3367 g sample of primary standard grade Na 2CO3 required 28.66-mL of a H 2SO4 solution to reach the endpoint. Calculate the following: Molar Masses: Na 2CO3 = 106.0 H 2SO4 = 98.08 a. Normality of the H 2SO4 solution ANSWER: 0.2217N H 2SO4 b. Molarity of the H 2SO4 solution ANSWER: 0.1108M H 2SO4 c. Na2CO3 titer of the acid ANSWER: 11.75-mg Na 2CO3 2. Exactly 50.0-mL of a HCl solution required 29.71-mL of 0.01963M Ba(OH) 2 to reach an endpoint with bromocresol green indicator. Calculate the Molarity and Normality Normality of the HCl solution. Molar Masses: HCl = 36.46 Ba(OH) 2 = 171.3 ANSWER : 0.0233M HCl / 0.0233N HCl 3. What is the percentage purity of acetic acid, if 3.0 grams require 20.5-mL of 0.105N NaOH solution to reach the endpoint? ANSWER: 4.3% CH3COOH Molar Masses: CH 3COOH = 60.05 NaOH = 40.00 4. A 0.6334-g sample of impure mercury (II) oxide was dissolved in an u unmeasured nmeasured excess of potassium iodide. Reaction: HgO(s) + 4 I- + H2O HgI 4-2 + 2 OHCalculate the percentage of HgO in the sample, if titration of the liberated hydroxide required 42.59 mL of 0.1178 M HCl. Molar Mass: HgO = 216.601 ANSWER: 85.58% HgO 5. Calculate the MgO content of a Milk of Magnesia sample, if a 12.32-g sample was dissolved in 50.0-mL of 1.034N H 2SO4 producing a mixture that required 24.6-mL of 1.125N NaOH. Molar Masses: MgO = 40.30 H 2SO4 = 98.08 NaOH = 40.00 ANSWER: 3.93% MgO 6. Calculate the alkaline strength of an impure sample of K 2CO3 in terms of percent K 2O from the following data. ANSWER: 61% K 2O Weight of sample 0.35-g Volume of HCl used 48.0-mL 1.0-mL HCl = 0.0053-g Na 2CO3 Volume of NaOH for back-titration 2.0-mL 1.0-mL NaOH = 0.02192-g KHC 2O4∙H2O Molar Masses: K 2CO3 = 138.2 K 2O = 94.20 Na 2CO3 = 106.0 KHC 2O4∙H2O = 146.1 7. A 0.4755-g sample containing (NH (NH 4)2C2O4 and inert materials was dissolved in water and made strongly alkaline with KOH, which converted NH 4+1 to NH 3. The liberated NH3 was distilled into exactly 50.0-mL of 0.05035M H 2SO4. The excess H2SO4 was back-titrated back-titrated with 11.13-mL of 0.1214M 0.1214M NaOH. Calculate the % Ammonium oxalate in the sample. ANSWER: 48.1% (NH 4)2C2O4 Molar Masses: (NH 4)2C2O4 = 124.1 H 2SO4 = 98.08 NaOH = 40.00 8. A sample of caustic soda weighing 1.675-g 1.675-g was found found to contain 97.25% of total alkali alkali calculated as NaOH, NaOH, of which 2.48% was Na 2CO3. What volume of 1.15N acid could have been consumed in the Pp and MO titrations as ordinarily performed? Molar Masses: NaOH = 40.00 Na 2CO3 = 106.0 ANSWER: mL Pp = 34.8-mL acid mL MO = 0.341-mL acid
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9. An alkaline sample sample of sodium sodium compounds weighing weighing 1.196-g was was dissolved in water, cooled cooled to 15 0C, treated with 2 drops of Pp indicator and titrated with 1.058N H 2SO4 requiring 6.8-mL to render the solution colorless. Upon adding 2 drops of MO and continuing the titration, 16.5-mL 16.5-mL more of the acid were needed to complete the analysis. analysis. What is the quantitative quantitative composition of the sample? Molar Masses: NaOH = 40.00 Na 2CO3 = 106.0 NaHCO 3 = 84.01 ANSWER: 64% Na 2CO3 & 72% NaHCO 3 10. A sample weighing weighing 1.675-g is suspected to be either KOH, K2CO3, KHCO3 or a mixture of these substances. Its aqueous solution was cooled to 10 0C and was titrated with 1.058M HCl requiring 17.7-mL to render solution colorless to Pp as indicator. On adding 2 drops of MO, the solution consumed further 6.0-mL of of the acid to complete the neutralization. Quantitatively interpret the composition composition of the sample. Molar Masses: KOH = 56.11 K 2CO3 = 138.2 KHCO 3 = 100.1 ANSWER: 41% KOH KOH & 52% K2CO3 11. A series of solutions solutions containing NaOH, Na2CO3 and NaHCO3, alone or in compatible combination, was titrated with 0.1202M HCl. The volumes of acid needed to titrate 25.00-mL portions portions of each solution to a (1) phenolphthalein and (2) bromocresol green endpoint are given in the table that follows. Use these information to deduce the composition of the solutions. solutions. In addition, calculate the number of milligrams milligrams of each solute per milliliter of solution. (1) (2) ANSWERS A 22.42 22.44 4.314-mg NaOH per mL B 15.67 42.13 7.985-mg Na 2CO3 per mL and 4.358-mg NaHCO 3 per mL C 29.64 36.42 3.455-mg Na 2CO3 per mL and 4.396-mg NaOH per mL D 16.12 32.23 8.215-mg Na 2CO3 per mL E 0.00 33.33 13.46-mg NaHCO 3 per mL PRACTICE SET: PRECIPITATION TITRATIONS
1. A standard solution is prepared by dissolving 8.3018-g of AgNO 3 in a one-liter volumetric flask. flask. Calculate the molar silver-ion concentration of this solution. Molar Mass: AgNO 3 = 169.9 ANSWER: 0.04887M AgNO 3 2. What is the molar concentration of a AgNO 3 solution if 16.35-mL reacts with a. 0.3017-g KIO 3 ANSWER: 0.08623M AgNO 3 b. 12.33-mL of 0.02149M K 4Fe(CN)6 ANSWER: 0.06482M AgNO 3 Molar Masses: KIO 3 = 214.0 K 4Fe(CN)6 = 368.4 3. A 100-mL sample of brackish water was made ammoniacal and the sulfide it contained titrated ti trated with 8.47 mL of 0.01301 M AgNO 3. The net reaction is 2 Ag+ + S-2 Ag2S (s) Calculate the parts per million of H 2S in the water. Molar Mass: H 2S = 34.05 ANSWER: 18.8 ppm H 2S 4. Titration of a 0.485-g sample by the Mohr method required 36.8 mL of standard 0.1060 M AgNO 3 solution. Calculate the percentage of chloride in the sample. Atomic Mass: Cl - = 35.45 ANSWER: 28.5% Cl -
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5. The chloride in a 0.2720-g sample was precipitated by introducing 50.0-mL of 0.1030M AgNO 3. Titration of the silver ion present in excess required 8.65-mL of 0.1260M 0.1260M KSCN. Express the result of the analysis in terms of percent MgCl 2. Molar Masses: MgCl 2 = 95.21 ANSWER: 71.1% MgCl 2 6. A two-liter sample of mineral water was evaporated to a small volume, following which the potassium ion was precipitated with excess sodium tetraphenylboron: K+1 + NaB(C 6H5)4 KB(C6H5)4 (s) The precipitate was filtered, washed washed and redissolved in acetone. The analysis was completed by a Mohr titration that required 43.85-mL of 0.03941M AgNO 3: KB(C6H5)4 (s) + Ag+1 AgB(C6H5)4 (s) + K+1 Calculate the potassium ion concentration (parts per million) of the water sample. Molar Mass: K = 39.10 ANSWER: 33.8-ppm K + PRACTICE SET: COMPLEX-FORMATION TITRATIONS
1. A solution was prepared by dissolving about 3.0 g of Na 2H2Y2H2O in approximately 1 L of water and standardizing against 50.00-mL aliquots of 0.004517 M Mg +2. An average titration of 32.22 mL mL was required. Calculate the molar concentration of the EDTA solution. ANSWER: 7.010 x 10-3 M EDTA 2. A 50.0-mL water sample was determined for hardness in terms of CaCO 3. It required 40.0-mL of of 0.0100M 0.0100M EDTA solution for titration. Calculate the total hardness in parts per million million (ppm) CaCO 3. Molar Mass: CaCO 3 = 100.1 ANSWER: 8.01 x 10 2 ppm CaCO 3 3. A 50.0-mL sample was titrated with 40.0-mL of disodium EDTA. (Note: 42.5mL EDTA solution was needed to titrate 0.08275-g MgO.) Another 50.0-mL portion of the water water sample was treated with K 2C2O4 and the resulting mixture consumed 10.0-mL of EDTA. Find the hardness in parts per million (ppm) due to CaCO 3. Molar Masses: MgO = 40.32 CaCO 3 = 100.1 ANSWER: 2.90 x 10 3 ppm CaCO 3 4. A sample of Al 2(SO4)3 weighing 7.52-g was dissolved in enough water and diluted to 250.0-mL. 100.0-mL of the dilution was transferred to another flask, 10.0-mL of 0.050M EDTA, 20-mL of buffer solution, 50-mL of alcohol and 2.0-mL dithizone dithizone TS were added. The resulting solution required 8.7-mL of of 0.031M ZnSO 4 to reach the endpoint. (Note: Each mL of 0.050M EDTA is equivalent to 16.66-mg Al 2(SO4)318H20). Compute for the percent purity of the sample. Molar Mass: Al 2(SO4)318H2O = 666.5 ANSWER: 2.5% Al 2(SO4)3
5. The silver ion in a 25.00-mL sample was converted to dicyanoargentate (l) ion by the addition of an excess solution containing Ni(CN)4-2: Ni(CN)4-2 + 2 Ag +1 2Ag(CN) 2-1 + Ni+2 The liberated nickel ion was titrated with 43.77-mL 43.77-mL of 0.02408M ED EDTA. TA. Calculate the molar concentration of the silver solution. Molar Masses: Ni = 58.69 Ag = 107.9 ANSWER: 0.08432M Ag +1
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6. A 0.3284-g sample of brass (containing lead, zinc, copper and tin) was dissolved in nitric acid. The sparingly soluble SnO24H2O was removed by filtration, and the combined filtrate and washings were diluted to 500.0-mL. A 10.00-mL aliquot was suitably buffered; titration of the lead, zinc and copper in this aliquot required 37.56-mL of 0.00250M EDTA. The copper in a 25.00-mL aliquot was masked with thiosulfate; the lead and zinc were then titrated with 27.67-mL of the EDTA solution. Cyanide ion was used to mask the copper and zinc in a 100.0-mL aliquot; 10.80-mL of the EDTA solution was needed to titrate the lead ion. Determine the composition of the brass sample. Molar Masses: Pb = 207.2 Zn = 65.37 Cu = 63.54 Sn = 118.7 ANSWERS: 8.517% Pb 24.85% Zn 64.07% Cu 2.56% Sn
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