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 a.n.a.lysis of silver coin (90% Ag), using a 0.5000 gram sample, what is the least normal value that a pota.s.sium thiocyanate solution may have and not require more than 50 cc. of solution in the a.n.a.lysis?
!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 t.i.trated 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 pota.s.sium 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 a.n.a.lYSIS
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.
77. If a sample of silver coin weighing 0.2500 gram gives a precipitate of AgCl weighing 0.2991 gram, what weight of AgI could have been obtained from the same weight of sample, and what is the percentage of silver in the coin?
!Answers!: 0.4898 gr.; 90.05%.
78. How many cubic centimeters of hydrochloric acid (sp. gr. 1.13 containing 25.75% HCl by weight) are required to exactly neutralize 25 cc. of ammonium hydroxide (sp. gr. .90 containing 28.33% NH_{3} by weight)?
!Answer!: 47.03 cc.
79. How many cubic centimeters of ammonium hydroxide solution (sp. gr.
0.96 containing 9.91% NH_{3} by weight) are required to precipitate the aluminium as aluminium hydroxide from a two-gram sample of alum (KAl(SO_{4})_{2}.12H_{2}O)? What will be the weight of the ignited precipitate?
!Answers!: 2.26 cc.; 0.2154 gram.
80. What volume of nitric acid (sp. gr. 1.05 containing 9.0% HNO_{3} by weight) is required to oxidize the iron in one gram of FeSO_{4}.7H_{2}O in the presence of sulphuric acid? 6FeSO_{4} + 2HNO_{3} + 3H_{2}SO_{4} = 3Fe_{2}(SO_{4})_{3} + 2NO + 4H_{2}O.
!Answer!: 0.80 cc.
81. If 0.7530 gram of ferric nitrate (Fe(NO_{3})_{3}.9H_{2}O) is dissolved in water and 1.37 cc. of HCl (sp. gr. 1.11 containing 21.92% HCl by weight) is added, how many cubic centimeters of ammonia (sp.
gr. 0.96 containing 9.91% NH_{3} by weight) are required to neutralize the acid and precipitate the iron as ferric hydroxide?
!Answer!: 2.63 cc.
82. To a suspension of 0.3100 gram of Al(OH)_{3} in water are added 13.00 cc. of aqueous ammonia (sp. gr. 0.90 containing 28.4% NH_{3} by weight). How many cubic centimeters of sulphuric acid (sp. gr. 1.18 containing 24.7% H_{2}SO_{4} by weight) must be added to the mixture in order to bring the aluminium into solution?
!Answer!: 34.8 cc.
83. How many cubic centimeters of sulphurous acid (sp. gr. 1.04 containing 75 grams SO_{2} per liter) are required to reduce the iron in 1 gram of ferric alum (KFe(SO_{4})_{2}.12H_{2}O)?
Fe_{2}(SO_{4})_{3} + SO_{2} + 2H_{2}O = 2FeSO_{4} + 2H_{2}SO_{4}.
!Answer!: 0.85 cc.
84. How many cubic centimeters of a solution of pota.s.sium bichromate containing 26.30 grams of K_{2}Cr_{2}O_{7} per liter must be taken in order to yield 0.6033 gram of Cr_{2}O_{3} after reduction and precipitation of the chromium?
K_{2}Cr_{2}O_{7} + 3SO_{2} + H_{2}SO_{4} = K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + H_{2}O.
!Answer!: 44.39 cc.
85. How many cubic centimeters of ammonium hydroxide (sp. gr. 0.946 containing 13.88% NH_{3} by weight) are required to precipitate the iron as Fe(OH)_{3} from a sample of pure FeSO_{4}.(NH_{4})_{2}SO_{4}.6H_{2}O, which requires 0.34 cc. of nitric acid (sp. gr. 1.350 containing 55.79% HNO_{3} by weight) for oxidation of the iron? (See problem No. 80 for reaction.)
!Answer!: 4.74 cc.
86. In the a.n.a.lysis of an iron ore by solution, oxidation and precipitation of the iron as Fe(OH)_{3}, what weight of sample must be taken for a.n.a.lysis so that each one hundredth of a gram of the ignited precipitate of Fe_{2}O_{3} shall represent one tenth of one per cent of iron?
!Answer!: 6.99 grams.
87. What weight in grams of impure ferrous ammonium sulphate should be taken for a.n.a.lysis so that the number of centigrams of BaSO_{4} obtained will represent five times the percentage of sulphur in the sample?
!Answer!: 0.6870 gram.
88. What weight of magnet.i.te must be taken for a.n.a.lysis in order that, after precipitating and igniting all the iron to Fe_{2}O_{3}, the percentage of Fe_{2}O_{4} in the sample may be found by multiplying the weight in grams of the ignited precipitate by 100?
!Answer!: 0.9665 gram.
89. After oxidizing the a.r.s.enic in 0.5000 gram of pure As_{2}S_{3} to a.r.s.enic acid, it is precipitated with "magnesia mixture" (MgCl_{2} + 2NH_{4}Cl). If exactly 12.6 cc. of the mixture are required, how many grams of MgCl_{2} per liter does the solution contain? H_{3}AsO_{4} + MgCl_{2} + 3NH_{4}OH = MgNH_{4}AsO_{4} + 2NH_{4}Cl + 3H_{2}O.
!Answer!: 30.71 grams.
90. A sample is prepared for student a.n.a.lysis by mixing pure apat.i.te (Ca_{3}(PO_{4})_{2}.CaCl_{2}) with an inert material. If 1 gram of the sample gives 0.4013 gram of Mg_{2}P_{2}O_{7}, how many cubic centimeters of ammonium oxalate solution (containing 40 grams of (NH_{4})_{2}C_{2}O_{4}.H_{2}O per liter) would be required to precipitate the calcium from the same weight of sample?
!Answer!: 25.60 cc.