10 Determ Oxygen In Air

HOW MUCH OXYGEN IS THERE IN THE AIR? Introduction Air is a mixture of gases mainly nitrogen and oxygen. Some other gases (basically argon) can be found in a small percentage. Carbon dioxide and water vapour are also found but the amount of them varies according to different circumstances (humidity, pollution, etc.). To determine the percentage of oxygen in the air you will make it react it with iron in a closed vessel. As iron gets oxidised it eats up the oxygen enclosed in the container. The total volume of air will decrease and you will measure the volume before the reaction starts and after it has finished. Iron wool can be easily oxidised in a weakly acidic moist ambient so you will add some acid to the water sealing the reaction vessel. Probably you will see how “rust” appears on the iron wool (brown stains) The word equation for the chemical reaction goes like this Iron + Oxygen → Iron oxide Aims To estimate the percentage (in volume) of oxygen in air Apparatus A 250 ml beaker, a small beaker, a measuring cylinder (or a boiling tube), a glass rod, a balance, a clamp, a retort stand, filter paper, a water proof marker. Iron wool, water and some acetic acid are going to be used. Procedure 1- Put some water (say 0,5 cm high) into the 250 ml beaker 2- Weigh approx. 1 g of iron wool in the balance 3- Half-fill the small beaker with dilute acetic acid (vinegar will also do). 4- Soak the iron wool in the acid solution for some 2 or 3 minutes to remove a black oxide that covers it, preventing the iron from getting oxidised. Use the glass rod to stir and move the iron wool so that the black oxide is more easily dissolved. 5- Now remove the iron wool from the acid and put it inside the tube (or measuring cylinder). 6- Remove excess acid from the iron wool with some filter paper (it should not be dried)

7- Push it to the bottom with the glass rod, taking care it is kept hanging loose so that air flows easily through. Work as fast as you can. 8- Working as fast as you can, put the tube (measuring cylinder) upside down into the 250 mol beaker, so that its mouth is sealed by water. This will prevent extra air from getting into the tube. If the piece of iron wool falls down to the water, repeat the experiment. 9- Consider holding the tube using a clamp bolted to a retort stand (optional). 10- Mark the water level in the tube at the beginning of the experiment. As oxygen is being used up, water will climb up the tube. 11- Add small amounts of water to the beaker so its level inside and outside the tube is the same. This will keep the pressure inside the tube equal to the atmospheric pressure so that conditions are kept unchanged and the experiment is valid. 12- Once water stops rising, mark its final level in the tube. 13- As the cross-section of the tube is reasonably constant, the volume of the air inside it is proportional to the length of the air column. 14- Call h0 the length of the air column before the experiment (initial length) and h the length of the air column after oxygen has been consumed. The “length” of oxygen that has “disappeared” (call it hox ) can be calculated : hox = h0 - h 15- The percentage of oxygen in air will be: %Oxygen = 100. hox / h0 16- Is your result reasonably coincident with the accepted value?