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126. How does oxygen occur in the free state and in the combined state in nature ?

127. What proportion of (a) air, (6) water, (c) the earth's crust, consists of oxygen ?

128. By whom was oxygen discovered, and in what year? Who discovered it independently, and when ?

129. What name was given to oxygen by its discoverer? Who proposed the name oxygen, and for what reasons ?

130. From what substance was oxygen first prepared ? Describe the form of apparatus originally used, and state how the experiment would be made now.

131. Express by equations the reactions which occur when (a) potassium chlorate, (6) mercuric oxide, (c) manganese dioxide, are strongly heated.

132. Why is manganese dioxide generally mixed with potassium chlorate for the preparation of oxygen ?

133. What impurity is always present in oxygen prepared by heating a mixture of potassium chlorate and manganese dioxide ? How may this impurity be removed ?

134. Name six oxides which when simply heated lose oxygen. Express each reaction by an equation.

135. How may oxygen be obtained from each of the following bodies ?-(a) water, (6) sulphuric acid, (c) potassium bichromate, (d) bleaching powder.

136. How may oxygen be obtained from potassium manganate and from potassium permanganate ?

137. Describe fully any process for obtaining oxygen from the air,

138. What processes have been proposed for obtaining continuous supplies of

oxygen from the air?

139. How may oxygen be obtained from (a) lime, (6) ferric oxide, (c) silica, (d) nitric acid ?

140. Give two distinct methods for the preparation of oxygen from water, and sketch the apparatus necessary for each.

141. Find the percentage of oxygen contained in each of the following substances :-(a) water, (6) potassium

chlorate, (c) manganese dioxide, (d) sulphuric acid, (e) mercuric oxide, (f) barium dioxide.

142. How much oxygen should be obtained from 100 lbs. of each of the above substances when used as sources of oxygen ?

143. How much mercuric oxide must be heated to yield one litre of oxygen at N.T.P. ?

144. Equal weights of manganese dioxide are heated (a) alone, (6) with sulphuric acid. Which process yields the greater amount of oxygen ?

145. Exactly 100 litres of oxygen at N.T.P. were obtained by heating potassium chlorate. How much of this salt was used ? 146. Some

pure manganese dioxide lost when strongly heated 137 grammes. Find the weight of the residue and the volume (at N.T.P.) of oxygen evolved.

147. A quantity of mercury was heated in air for several days, and gained in weight 134 grammes. Find the volume of oxygen at N.T.P. removed from the air.

148. Four hundred and seventy-nine grammes of potassium chloride remained after heating some potassium chlorate. Find the weight of the latter salt used.

149. A glass flask filled with air at N.T.P. weighs 52 grammes. What capacity must it have so that when filled with oxygen at 14° C. and 753 m.m. pressure, it shall weigh 53:47 grammes ?

150. What are the most important properties of oxygen? How is it distinguished from other gases ?

151. How did Pictet obtain the gas in the liquid state and ascertain its density ?

152. The following bodies are separately burned in oxygen, (a) hydrogen, (6) sulphur, (c) charcoal, (d) iron, (e) phosphorus, (f) magnesium. Give the name of the oxide formed in each case, and express all the reactions by equations.

153. Explain the terms “combustion,” “oxidation,” “reduction,” and “ deoxidation.”

154. Sulphur is burnt in a jar of air, and also in a jar of oxygen.

Name the gases present in each jar after the combustion, and state the action of water on each.

155. Why was the name oxygen given to the gas, and why is it inappropriate ? To what element would the name more suitably apply?

156. What chemical changes occur when a candle burns in oxygen? How may each of the products be recognised?

157. At the bottom of a glass flask some sand is placed and on the top of it a fragment of phosphorus. The flask is then filled with dry oxygen, closed air-tight, and the whole weighed. The flask is then warmed until the phosphorus kindles, and when quite cold re-weighed. The stopper is removed for an instant and a third weighing made. How will the three weighings differ?

158. If air were used instead of oxygen in the previous question, what difference would be noticed in the weighings?

159. By burning a candle in oxygen 13:21 grammes carbon dioxide and 5'58 grammes water were produced. What loss in weight did the candle suffer, and what volume of oxygen at N.T.P. was required for the combustion ?

160. What weight of oxygen is contained in a room measuring 23 feet long, 17 broad, and 13 high? With how much phosphorus would this quantity of oxygen combine?

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