Inorganic chemistry and the periodic table

Chemistry B (Salters) - H033 · Cambridge OCR AS Level · 4 min read · Updated: 26 Mar 2026

Welcome to The Ozone Story: Measuring the Atmosphere

In this part of The Ozone Story (OZ), we are looking at how scientists measure the different gases in our atmosphere. While we usually think of air as just "oxygen," it’s actually a complex mixture of many different gases. Some of these gases are present in huge amounts, while others—like ozone or pollutants—are present in tiny, tiny quantities. Because these amounts are so small, we need a special way to talk about them so the numbers don't get confusing!

Don’t worry if the math seems a bit intimidating at first. Once you see the pattern, it’s as simple as moving a decimal point. Let’s dive in!

1. Measuring Gas by Volume

When we talk about gas mixtures in chemistry, we usually measure them by volume. This is because, at the same temperature and pressure, equal volumes of different gases contain the same number of particles (this is a prerequisite concept known as Avogadro’s Law).

To describe how much of a specific gas is in the air, we use two main scales:

1. Percentage Concentration (%): Parts per hundred. We use this for gases that make up a large part of the air (like Nitrogen and Oxygen).
2. Parts Per Million (ppm): We use this for "trace" gases that are present in very small amounts (like Carbon Dioxide, Methane, or Ozone).

Quick Review: The Big Three
The atmosphere is mostly made of:
• Nitrogen ($N_2$): ~78%
• Oxygen ($O_2$): ~21%
• Argon ($Ar$): ~0.9%

2. Understanding Parts Per Million (ppm)

The term parts per million sounds fancy, but it just means "how many units of this gas are there for every million units of total air?"

An Everyday Analogy:
Imagine you have a giant jar containing 1,000,000 white marbles. If you swap 1 white marble for 1 red marble, the concentration of red marbles is 1 ppm.
• 1 ppm is like one minute in two years.
• 1 ppm is like one cent in $10,000.

Did you know? Even though 1 ppm sounds like almost nothing, for some gases, it’s a huge amount! For example, the concentration of ozone in the "ozone layer" is only about 8 to 12 ppm, yet it's enough to protect us from dangerous UV radiation.

3. How to Calculate Concentrations

The syllabus requires you to calculate these values from given data. Here are the two formulas you need to know:

A. Calculating ppm from Volume

$\text{Concentration in ppm} = \frac{\text{Volume of the gas component}}{\text{Total volume of the gas mixture}} \times 1,000,000$

B. Converting Percentage (%) to ppm

Since "percent" means "parts per 100" and "ppm" means "parts per 1,000,000", the difference between them is a factor of 10,000 (because $1,000,000 \div 100 = 10,000$).

The Golden Rule:
• To go from % to ppm: Multiply by 10,000.
• To go from ppm to %: Divide by 10,000.

Memory Aid:
Think of "P.M." (like the afternoon). Percent to Million = Multiply!

4. Step-by-Step Example

Scenario: A sample of air contains 0.04% Carbon Dioxide ($CO_2$). What is this concentration in ppm?

Step 1: Identify your starting value (0.04%).
Step 2: Use the Golden Rule (Multiply by 10,000).
Step 3: $0.04 \times 10,000 = 400$.
Answer: The concentration is 400 ppm.

Scenario 2: An ozone sensor reads 5 ppm. What is this as a percentage?
Step 1: Identify your starting value (5 ppm).
Step 2: Use the Golden Rule (Divide by 10,000).
Step 3: $5 \div 10,000 = 0.0005$.
Answer: The concentration is 0.0005%.

5. Common Mistakes to Avoid

• Decimal Point Errors: When multiplying by 10,000, you move the decimal point 4 places to the right. When dividing, move it 4 places to the left. Always double-check your zeroes!
• Mixing Units: Ensure the "Volume of component" and "Total volume" are in the same units (e.g., both in $cm^3$ or both in $dm^3$) before you divide them.
• The "Total" Volume: Make sure you are dividing by the total volume of the mixture, not just the volume of the other gases.

Key Takeaways

Quick Review Box

• Percentage = parts per 100.
• ppm = parts per 1,000,000.
• Conversion: $1\% = 10,000 \text{ ppm}$.
• Atmospheric context: We use ppm for trace gases like ozone and pollutants because using percentages would result in too many tiny decimals (like 0.00001%), which are hard to read and compare.

You've got this! Being able to switch between ppm and percentage is a core skill for the OZ section. Practice a few decimal moves, and it will become second nature.

References

AS and A Level Chemistry B (Salters) Specification — Oxford Cambridge and RSA Examinations (OCR)
OCR AS and A Level Chemistry B Delivery Guide – Formulae, equations and amount of substance — Oxford Cambridge and RSA Examinations (OCR)
OCR Chemistry B (Salters) Specification at a Glance — Oxford Cambridge and RSA Examinations (OCR)

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

Why are gas mixtures in the atmosphere typically measured by volume rather than mass?

At the same temperature and pressure, equal volumes of different gases contain the same number of particles (Avogadro’s Law).

What are the three most abundant gases in the Earth's atmosphere and their approximate percentages?

Nitrogen ($N_2$) at ~78%, Oxygen ($O_2$) at ~21%, and Argon ($Ar$) at ~0.9%.

What does the concentration unit 'parts per million' (ppm) represent?

It represents the number of units of a specific gas component present for every one million units of the total gas mixture.

How do you calculate a gas concentration in ppm from volume data?

$\text{Concentration in ppm} = \frac{\text{Volume of the gas component}}{\text{Total volume of the gas mixture}} \times 1,000,000$

What is the mathematical relationship between 1% and 1 ppm?

A concentration of 1% is equal to 10,000 ppm.

What is the rule for converting a gas concentration from percentage (%) to ppm?

Multiply the percentage value by 10,000.

What is the rule for converting a gas concentration from ppm to percentage (%)?

Divide the ppm value by 10,000.

When calculating gas ratios, why is the 'Total Volume' value critical?

You must divide by the total volume of the entire mixture, not just the volume of the other gases present.

Why is ppm used for trace gases like ozone instead of percentage?

It avoids using cumbersome, tiny decimals that are difficult to read and compare.

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