Finding the Proportion of Oxygen in Air

Introduction

How do we know that air is about 21% oxygen? You cannot see the oxygen or pick it out of the air directly. The trick, used since the eighteenth century, is to remove the oxygen by making it react with a solid, and then measure how much the volume of the gas shrinks. This page explains the idea, two classic methods, how to calculate the result, and why real results are often slightly below 21%.

Core explanation

The principle. Take a measured volume of air in a sealed apparatus. Let it react with a substance that combines only with oxygen and forms a solid product. The oxygen molecules are removed from the gas and locked into the solid. Nitrogen, argon and the other gases do not react, so they remain. The fall in gas volume equals the volume of oxygen that was present:

percentage of oxygen = (decrease in volume ÷ original volume) × 100

Method 1: heated copper. A known volume of air, often 100 cm³, is held in a gas syringe connected through a tube containing copper. The copper is heated and the air is pushed back and forth over it many times. The shiny pink-brown copper turns black as copper(II) oxide forms:

copper + oxygen → copper(II) oxide 2Cu + O₂ → 2CuO

The volume falls until it stays constant, showing that all the oxygen has reacted. The apparatus is then allowed to cool back to room temperature, because warm gas expands and would give a misleading reading. A typical final volume is about 79 cm³, so about 21 cm³ of oxygen was present: 21%.

Method 2: rusting iron. Damp iron wool is pushed into the closed end of a measuring cylinder or tube, which is then stood upside down in water. Over about a week the iron slowly rusts, using up the oxygen in the trapped air. Water rises up the tube to take the place of the oxygen. When the water level stops rising, it has risen by about one-fifth of the original air volume.

iron + oxygen + water → hydrated iron(III) oxide (rust)

This method needs no heating, but it is slow.

Why the copper must be in excess. There must be more copper (or iron) than the oxygen can react with. If the solid ran out first, some oxygen would be left unreacted and the measured percentage would be too low.

What the remaining gas shows. The gas left over does not relight a glowing splint and does not support burning, showing that the oxygen really has gone. It is mostly nitrogen, with about 1% argon.

Sources of error. Real results are often a little below 21%, for example 18–20%. Common reasons are:

- not passing the air over the copper enough times, so some oxygen remains; - reading the volume while the gas is still warm, making the final volume seem too large; - leaks at the joints of the apparatus; - in the iron method, not waiting long enough for rusting to finish.

Step-by-step reasoning

To calculate the percentage of oxygen from results:

1. Record the starting volume of air. 2. Record the final volume after the reaction has finished and the gas has cooled to room temperature. 3. Subtract to find the decrease in volume — this is the volume of oxygen. 4. Divide the decrease by the starting volume and multiply by 100.

Visual explanation

Imagine two gas syringes joined by a glass tube of copper turnings. At the start, one syringe reads 100 cm³ and the copper is shiny. After many passes, the copper nearest the entry has turned black and the plungers together read only 79 cm³. The 21 missing cm³ of gas is now part of the black solid in the tube.

Real-world analogy

It is like finding how many red sweets are in a jar of mixed sweets by letting a friend who only eats red ones help themselves until none are left. The number of sweets that disappeared tells you how many were red, without ever sorting the jar yourself.

Real-world example

The same principle — remove one gas and measure the change — is used in simple oxygen absorbers packed with food such as beef jerky. A small sachet of iron powder reacts with the oxygen in the sealed packet, protecting the food from spoiling. The packet often visibly shrinks inwards as the oxygen is removed.

Why?

Why must the gas be cooled before its final volume is read? Heating makes gas particles move faster and spread out, so a hot gas takes up more volume. If the final volume were read while hot, it would be too large, the decrease would seem too small, and the calculated oxygen percentage would be too low.

Common misconception

"The volume goes down because the air is burnt up." The nitrogen and argon are unchanged and still there. Only the oxygen is removed, and it has not vanished — it has combined with the copper or iron to form a solid oxide, so the mass of the solid increases.

Worked example

Question: A student passed 90 cm³ of air over hot copper. After cooling, 72 cm³ of gas remained. Calculate the percentage of oxygen and comment on the result.

Reasoning: Decrease = 90 − 72 = 18 cm³. Percentage = (18 ÷ 90) × 100 = 20%.

Answer: 20%. This is slightly below the accepted 21%, probably because not all the oxygen reacted or the gas was still slightly warm.

Quick check

1. Why does the copper turn black during the experiment? Answer: It reacts with oxygen to form black copper(II) oxide.

Exam focus

Examiners often give syringe readings and ask for the percentage of oxygen, so practise the calculation. Explain why you pass the air over the copper repeatedly, why you cool before reading, and why the copper must be in excess. Give the equation 2Cu + O₂ → 2CuO.

Advanced insight

Other reactive substances could remove oxygen, but they must form a solid or a product that dissolves, not a gas. Burning a candle in a jar over water is a poor method: the candle produces carbon dioxide and water vapour, the heated air expands and escapes, and the flame goes out before all the oxygen is used, so the "answer" is unreliable.

Summary

The percentage of oxygen in air is found by reacting the oxygen away with a substance, such as heated copper or damp iron wool, that forms a solid product. The decrease in gas volume equals the volume of oxygen. Results close to 21% are obtained if the solid is in excess, the reaction goes to completion, there are no leaks and gas volumes are measured at room temperature.

Practice questions

1. Write the balanced equation for the reaction of copper with oxygen. Answer: 2Cu + O₂ → 2CuO. 2. 100 cm³ of air is passed over hot copper; 80 cm³ remains after cooling. What percentage of the air was oxygen? Answer: (20 ÷ 100) × 100 = 20%. 3. Give two reasons why a student's result might be lower than 21%. Answer: Not all the oxygen reacted (too few passes or not enough copper), the gas was measured while still warm, or there was a leak. 4. In the iron wool method, why does water rise up the tube? Answer: The iron uses up the oxygen as it rusts, reducing the pressure of the trapped gas, so atmospheric pressure pushes water up to take the place of the oxygen.