!Answer!:  1090 cc.

63.  An analyzed sample of stibnite containing 70.05% Sb is given for analysis.  A student titrates it with a solution of iodine of which 1 cc. is equivalent to 0.004950 gram of As_{2}O_{3}.  Due to an error on his part in standardization, the student’s analysis shows the sample to contain 70.32% Sb.  Calculate the true normal value of the iodine solution, and the percentage error in the analysis.

!Answers!:  0.1000 N; 0.39%.

64.  A sample of pyrolusite weighing 0.5000 gram is treated with an excess of hydrochloric acid, the liberated chlorine is passed into potassium iodide and the liberated iodine is titrated with sodium thiosulphate solution (49.66 grams of pure Na_{2}S_{2}O_{3}.5H_{2}O per liter).  If 38.72 cc. are required, what volume of 0.25 normal permanganate solution will be required in an indirect determination in which a similar sample is reduced with 0.9012 gram H_{2}C_{2}O_{4}.2H_{2}O and the excess oxalic acid titrated?

!Answer!:  26.22 cc.

65.  In the determination of sulphur in steel by evolving the sulphur as hydrogen sulphide, precipitating cadmium sulphide by passing the liberated hydrogen sulphide through ammoniacal cadmium chloride solution, and decomposing the CdS with acid in the presence of a measured amount of standard iodine, the following data are obtained:  Sample, 5.027 grams; cc.  Na_{2}S_{2}O_{3} sol. = 12.68; cc.  Iodine sol. = 15.59; 1 cc.  Iodine sol. = 1.086 cc.  Na_{2}S_{2}O_{3} sol.; 1 cc.  Na_{2}S_{2}O_{3}= 0.005044 gram Cu.  Calculate the percentage of sulphur. (H_{2}S + I_{2} = 2hi + S.)

!Answer!:  0.107%.

66.  Given the following data, calculate the percentage of iron in a sample of crude ferric chloride weighing 1.000 gram.  The iodine liberated by the reaction 2FeCl_{3}+ 2hi = 2HCl + 2FeCl_{2} + I_{2} is reduced by the addition of 50 cc. of sodium thiosulphate solution and the excess thiosulphate is titrated with standard iodine and requires 7.85 cc. 45 cc.  I_{2} solution = 45.95 cc.  Na_{2}S_{2}O_{3} solution; 45 cc.  As_{2}O_{3} solution = 45.27 cc.  I_{2} solution. 1 cc. arsenite solution = 0.005160 gram As_{2}O_{3}.

!Answer!:  23.77%.

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?