(2)n= m M = V V m = N N A It can be known that the smaller the molar mass of gas, the more substances, the more molecules and the larger the volume. Among the five gases with the same mass, H 2, NH 3, Cl 2, SO 2 and O 3, the smallest molar mass of hydrogen leads to the largest number of molecules, while the largest molar mass of chlorine leads to the smallest number and volume of substances.

So, the answer is: H 2, cl 2；;

(3) (1) concentrated sulfuric acid is often used as a desiccant for some gases, which makes use of the water absorption of concentrated sulfuric acid.

When copper and concentrated sulfuric acid are heated, the gas is released as sulfur dioxide gas, and copper sulfate is generated at the same time, which makes use of the strong oxidation and acidity of concentrated sulfuric acid.

When concentrated sulfuric acid is added to sucrose, sucrose turns black, which makes use of the dehydration of concentrated sulfuric acid.

So the answer is: ① water absorption; ② Strong oxidation and acidity; ③ dehydration;

(4) Iron is located in Group VIII of the Periodic Table of Elements, and the most active nonmetallic elements in the Periodic Table of Elements are located in Group VIII, in column 17. At present, the period of 32 elements in the periodic table is the sixth period, including lanthanides.

So the answer is: ⅷ; 17; Six.