67. Sulphide sulphur was determined in a sample of reduced barium sulphate by the evolution method, in which the sulphur was evolved as hydrogen sulphide and was passed into CdCl_{2} solution, the acidified precipitate being titrated with iodine and thiosulphate. Sample, 5.076 grams; cc. I_{2} = 20.83; cc. Na_{2}S_{2}O_{3} = 12.37; 43.45 cc. Na_{2}S_{2}O_{3} = 43.42 cc. I_{2}; 8.06 cc. KMnO_{4} = 44.66 cc. Na_{2}S_{2}O_{3}; 28.87 cc. KMnO_{4} = 0.2004 gram Na_{2}C_{2}O_{4}. Calculate the percentage of sulphide sulphur in the sample. !Answer!: 0.050%. 68. What weight of pyrolusite containing 89.21% MnO_{2} will oxidize the same amount of oxalic acid as 37.12 cc. of a permanganate solution, of which 1 cc. will liberate 0.0175 gram of I_{2} from KI? !Answer!: 0.2493 gram. 69. A sample of pyrolusite weighs 0.2400 gram and is 92.50% pure MnO_{2}. The iodine liberated from KI by the manganese dioxide is sufficient to react with 46.24 cc. of Na_{2}S_{2}O_{3} sol. What is the normal value of the thiosulphate? !Answer!:: 0.1105 N. 70. In the volumetric analysis of silver coin (90% Ag), using a 0.5000 gram sample, what is the least normal value that a potassium thiocyanate solution may have and not require more than 50 cc. of solution in the analysis? !Answer!: 0.08339 N. 71. A mixture of pure lithium chloride and barium bromide weighing 0.6 gram is treated with 45.15 cubic centimeters of 0.2017 N silver nitrate, and the excess titrated with 25 cc. of 0.1 N KSCN solution, using ferric alum as an indicator. Calculate the percentage of bromine in the sample. !Answer!: 40.11%. 72. A mixture of the chlorides of sodium and potassium from 0.5000 gram of a feldspar weighs 0.1500 gram, and after solution in water requires 22.71 cc. of 0.1012 N silver nitrate for the precipitation of the chloride ions. What are the percentages of Na_{2}O and K_{2}O in the feldspar? !Answer!: 8.24% Na_{2}O; 9.14% K_{2}O. GRAVIMETRIC ANALYSIS 73. Calculate (a) the grams of silver in one gram of silver chloride; (b) the grams of carbon dioxide liberated by the addition of an excess of acid to one gram of calcium carbonate; (c) the grams of MgCl_{2} necessary to precipitate 1 gram of MgNH_{4}PO_{4}. !Answers!: (a) 0.7526; (b) 0.4397; (c) 0.6940. 74. Calculate the chemical factor for (a) Sn in SnO_{2}; (b) MgO in Mg_{2}P_{2}O_{7}; (c) P_{2}O_{5} in Mg_{2}P_{2}O_{7}; (d) Fe in Fe_{2}O_{3}; (e) SO_{4} in BaSO_{4}. !Answers!: (a) 0.7879; (b) 0.3620; (c) 0.6378; (d) 0.6990; (e) 0.4115. 75. Calculate the log factor for (a) Pb in PbCrO_{4}; (b) Cr_{2}O_{3} in PbCrO_{4}; (c) Pb in PbO_{2} and (d) CaO in CaC_{2}O_{4}. !Answers!: (a) 9.8069-10, (b) 9.3713-10; (c) 9.9376-10; (d) 9.6415-10. 76. How many grams of Mn_{3}O_{4} can be obtained from 1 gram of MnO_{2}? !Answer!: 0.8774 gram.