69. (a) (i) Name the catalyst which helps in the conversion of sulphur dioxide to sulphur trioxide in step C.   70. (ii) In the Contact process for the manufacture of sulphuric acid, sulphur trioxide is not converted to sulphuric acid by reacting it with water. Instead a two step procedure is used. Write the equations for the two steps involved in D.   71. (iii) What type of substance will liberate sulphur dioxide from sodium sulphite in step E?   72. (iv) Write the equation for the reaction by which sulphur dioxide is converted to sodium sulphite in step F.   73. (b) (i) Write the equation for the preparation of ethylene from ethyl alcohol.   74. (ii) Write the general formula for a saturated hydrocarbon and give one example of a saturated hydrocarbon with its structural formula.   75. (iii) Name a compound which will give acetylene gas when treated with water.   76. (iv) Which particular property of cast iron makes it unsuitable for the construction of bridges?