Chemistry

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Average Atomic Mass Calculator.

Calculate weighted average atomic mass for two isotopes.

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01

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Average atomic mass: 12.011 u

Average atomic mass

12.011000u

Educational formula calculator. Keep units consistent; chemistry procedures, concentrations, and safety decisions require qualified laboratory guidance. Never mix cleaning products.

Use Cases

Chemistry Homework Help

Quickly verify your manual calculations for average atomic mass problems involving two isotopes, saving time and reducing errors.

Example: Check the average atomic mass of chlorine given Cl-35 (75%) and Cl-37 (25%).

Isotope Mixture Analysis

Estimate the average atomic mass of a sample when you know the masses and relative abundances of two isotopes, useful for lab work or study.

Example: Determine the average mass of a boron sample with B-10 (20%) and B-11 (80%).

Frequently Asked Questions

How do I calculate average atomic mass with this calculator?
Enter the mass (in atomic mass units, u) and abundance (as a percentage) for each of the two isotopes. The calculator multiplies each mass by its abundance (converted to a fraction) and sums the results to give the weighted average atomic mass.
What if the abundances don't add up to 100%?
The calculator uses the abundances you enter as they are. For a correct average, the two abundances should sum to 100%. If they don't, the result will be skewed. Ensure your inputs are accurate.
Can I use this for elements with more than two isotopes?
No, this calculator is designed for exactly two isotopes. For elements with more isotopes, you would need a tool that accepts multiple entries or calculate manually by summing mass*abundance for all isotopes.

Tips & Common Mistakes

Tips

  • Ensure the abundance percentages add up to 100% for accurate results.
  • Use the atomic mass unit (u) consistently for both isotope masses.
  • Double-check your inputs for typos, especially decimal points in mass values.
  • Remember that the result is a weighted average, so the more abundant isotope influences the final value more.

Common Mistakes to Avoid

  • Entering abundance as a decimal (e.g., 0.75) instead of a percentage (75).
  • Using masses in grams instead of atomic mass units (u).
  • Forgetting to convert percentages to fractions; the calculator does this automatically, but ensure you input the percentage value as given.

Last updated: August 13, 2026