8. How much lead peroxide could be obtained from 1 kg. of minium? 9. The purity of white lead is usually determined by observing the volume of carbon dioxide given off when it is treated with an acid. What acid should be used? On the supposition that it has the formula 2PbCO_{3}.Pb(OH)_{2}, how nearly pure was a sample if 1 g. gave 30 cc. of carbon dioxide at 20 deg. and 750 mm.? 10. Silicon belongs in the same family with tin and lead. In what respects are these elements similar? 11. What weight of tin could be obtained by the reduction of 1 ton of cassiterite? 12. What reaction would you expect to take place when lead peroxide is treated with hydrochloric acid? 13. White lead is often adulterated with barytes. Suggest a method for detecting it, if present, in a given example of white lead. CHAPTER XXX MANGANESE AND CHROMIUM ==================================================================== | | | | | | SYMBOL | ATOMIC | DENSITY | MELTING | FORMULAS OF ACIDS | | WEIGHT | | POINT | __________|________|________|_________|_________|___________________ | | | | | Manganese | Mn | 55.0 | 8.01 | 1900 deg. | H_{2}MnO_{4} and | | | | | HMnO_{4} Chromium | Cr | 52.1 | 7.3 | 3000 deg. | H_{2}CrO_{4} and | | | | | H_{2}Cr_{2}O_{7} ==================================================================== ~General.~ Manganese and chromium, while belonging to different families, have so many features in common in their chemical conduct that they may be studied together with advantage. They differ from most of the elements so far studied in that they can act either as acid-forming or base-forming elements. As base-forming elements each of the metals forms two series of salts. In the one series, designated by the suffix "ous," the metal is divalent; in the other series, designated by the suffix "ic," the metal is trivalent. Only the manganous and the chromic salts, however, are of importance. The acids in which these elements play the part of a non-metal are unstable, but their salts are usually stable, and some of them are important compounds. MANGANESE ~Occurrence.~ Manganese is found in nature chiefly as the dioxide MnO_{2}, called pyrolusite. In smaller amounts it occurs as the oxides Mn_{2}O_{3} and Mn_{3}O_{4}, and as the carbonate MnCO_{3}. Some iron ores also contain manganese. ~Preparation and properties.~ The element is difficult to prepare in pure condition and has no commercial applications. It can be prepared, however, by reducing the oxide with aluminium powder or by the use of the electric furnace, with carbon as the reducing agent. The metal somewhat resembles iron in appearance, but is harder, less fusible, and more readily acted upon by air and moisture. Acids readily dissolve it, forming manganous salts.