Figuring out the volume of air trapped inside a bottle of water I am doing a chemistry lab report on figuring out the volume of air trapped inside a bottle.  Here is the procedure: 


Fill a larger container or tub about half full with tap water
Fill a 2 L plastic soft drink bottle with tap water.  Put the cap on loosely
Carefully turn the bottle upside down without spilling any water
Put the bottle into the large container of water so that the mouth of the bottle is underwater.  Remove the cap underwater
Feed the flexible tubing under the water so that one end goes inside the bottle
Put your straw into the other end of the tubing.  Do not share straws
When it is your turn, exhale into the straw to collect the air of one normal breath
With a marking pen, mark the volume of air on the soft drink bottle
Figure out the volume of the air trapped inside the bottle.  Record this volume


How might I do number 9? My proposed procedure is:


With a marking pen, mark the new volume of air on the soft drink
bottle
Record this volume, which will be smaller than the volume found in
step 8 as some of the water will have been displaced by the breath
Subtract the new volume of air from the old volume of air to find
the volume of the air trapped inside the bottle
Record this volume


Thanks!
Displacement! Let the container that you have bottle in be filled to the brim inside a bigger container. Then when you breathe into the bottle, the amount of water that is displaced will be the same as the volume of the air.