Answer:
Air is a mixture. Its constituents can be separated. For example: oxygen, nitrogen etc.
Hydrochloric acid is a mixture, being an acidic liquid.
Answer:
The exposed metal rusts is an example of a chemical change because rust is an example of a chemical change in objects for example bicycles, scooters, etc.
Answer:
77.14 atm of pressure should be of an acetylene in the tank.
Explanation:

According to reaction, 2 moles of acetylene reacts with 5 moles of oxygen.
Moles of oxygen=
Moles of acetylene =

Volume of large tank with oxygen gas, 
Pressure of the oxygen gas inside the tank = 
..[1]
Volume of small tank with acetylene gas ,
Pressure of the acetylene gas inside the tank = 
..[2]
Considering both the gases having same temperature T, [1]=[2]



77.14 atm of pressure should be of an acetylene in the tank.