Avogadro's Number Examples in Chemistry

Start with the recap, study the fully worked examples, then use the practice problems to check your understanding of Avogadro's Number.

This page combines explanation, solved examples, and follow-up practice so you can move from recognition to confident problem-solving in Chemistry.

Concept Recap

The defined number of particles in exactly one mole of any substance: 6.022 \times 10^{23}.

A mind-bogglingly large number — but it's exactly the right size to make atomic counting practical.

Read the full concept explanation →

How to Use These Examples

  • Read the first worked example with the solution open so the structure is clear.
  • Try the practice problems before revealing each solution.
  • Use the related concepts and background knowledge badges if you feel stuck.

What to Focus On

Core idea: This specific number makes 1 mole of C-12 atoms weigh exactly 12 grams.

Common stuck point: Avogadro's number is exact—it's the definition, not a measurement.

Sense of Study hint: When a problem gives you moles and asks for particles, multiply by 6.022 \times 10^{23}. First identify whether you need atoms, molecules, or formula units. Then multiply or divide by N_A depending on the conversion direction. Finally, check your units cancel correctly.

Worked Examples

Example 1

easy
How many molecules are in 2.5 mol of \text{CO}_2?

Solution

  1. 1
    Convert moles to particles with Avogadro's number: N = n \times N_A.
  2. 2
    Substitute the values: N = 2.5 \times 6.022 \times 10^{23}.
  3. 3
    Multiply to get N = 1.506 \times 10^{24} molecules, which rounds to 1.51 \times 10^{24} molecules.

Answer

1.51 \times 10^{24}\text{ molecules}
Avogadro's number (N_A = 6.022 \times 10^{23}\,\text{mol}^{-1}) converts between moles and individual particles. It works for atoms, molecules, ions, or any countable entity.

Example 2

medium
How many individual oxygen atoms are in 1.5 mol of \text{H}_2\text{O}?

Example 3

medium
How many molecules are in 2.5 moles of water (\text{H}_2\text{O})? How many individual atoms is that?

Practice Problems

Try these problems on your own first, then open the solution to compare your method.

Example 1

easy
How many atoms are in 0.50 mol of iron (Fe)?

Example 2

medium
How many chloride ions are in 0.200 mol of \text{CaCl}_2?
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Related Concepts

MoleMolar Mass

Background Knowledge

These ideas may be useful before you work through the harder examples.

mole