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 potassium 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.