Rank the four gases (air, exhaled air, gas produced from the decomposition of h2o2, gas from decomposition of nahco3) in order of decreasing concentration of oxygen). give observational evidence for your rankings.

Gas produced from the decomposition of H2O2 
air 
exhaled air 
gas from decomposition of NaHCO3  
When H2O2 decomposes, you get oxygen gas and water, so the result should be close to 100% oxygen and you can't get higher than that. The next lower will be regular air at a concentration of approximately 21%. After that will be exhaled air because the human body uses some of the oxygen in the air for it's biological energy needs. And finally, last place goes to the decomposition of NaHCO3 which breaks down into sodium carbonate, water and carbon dioxide.  
There are some simple experiments you can perform to show the relative concentration of oxygen. 
1. gas produced from the decomposition of H2O2 
Take a wooden splint and ignite it until you get glowing coal at the end. Blow out the splint so there's no longer any flame, but the coal is still glowing. Insert the glowing splint into the gas produced by the decomposition of H2O2, whereupon the splint should burst back into flame.  
2 & 3. Air and exhaled air. 
You can do the wet steel wool experiment. For both gases, take a piece of steel wool dip it in water, then stuff the damp steel wool into the end of a 100 mL graduated cylinder. Invert the cylinder into a cup of water, allowing for a little of the gas to escape so that the water will be a few ml into the cylinder. Then note the volume of gas in the cylinder. Once a day, take note of the new gas level until the gas level stops changing. From there, you can determine the approximate percentage of oxygen that was in the air.  
4. decomposition of NaHCO3 
Place the gas in a beaker and then insert a burning splint into the beaker. The splint should be immediately extinguished since there is no oxygen in the beaker.