Gases in the Atmosphere

Principle

• A reactive material is sealed inside a fixed volume of air and allowed to combine with the oxygen in that air
• The reacted oxygen leaves the gas phase, so the trapped gas volume shrinks
• The change in volume, divided by the starting volume, gives the percentage of oxygen

Method: iron in a burette

• Push damp iron wool into the closed lower end of an inverted burette, so the iron has plenty of contact with water and air
• Stand the burette mouth-down in a trough of water, with the tap closed at the top
• Read the water level inside the burette against the scale and write it down as the initial reading
• Leave the apparatus for several days while the iron slowly rusts, using up oxygen from the trapped air
• Once the water level stops rising, take a second reading against the same scale

Sample calculation (worked through)

• Suppose the trapped air column measured 45.0 cm³ at the start, and 36.0 cm³ once rusting had stopped
• The volume of oxygen used up = 45.0 − 36.0 = 9.0 cm³
• Percentage of O₂ in the original air = (9.0 ÷ 45.0) × 100 = 20.0%
• This is close to the accepted value near 21%; the small shortfall is because rusting is slow and some oxygen may remain unreacted

Alternative material: burning phosphorus

• Phosphorus burns readily in a sealed bell jar over water, consuming oxygen quickly rather than over days
• The water rises into the jar as the trapped gas contracts, by the same volume principle
• Phosphorus is toxic, so the iron-wool method is normally preferred for school work