The weighted atomic mass is calculated by summing the products of each isotope's mass and its relative abundance (expressed as a decimal).
Here's a breakdown of the process:
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Identify the Isotopes: Determine all the isotopes of the element you're interested in. Isotopes are atoms of the same element that have different numbers of neutrons.
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Find the Mass of Each Isotope: The mass of each isotope is typically given in atomic mass units (amu). This information can usually be found in a periodic table or a table of isotopes.
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Determine the Relative Abundance of Each Isotope: The relative abundance represents the percentage of each isotope found naturally. This is often expressed as a percentage and must be converted to a decimal by dividing by 100. For example, an abundance of 75% becomes 0.75.
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Multiply Mass by Abundance: Multiply the mass of each isotope by its relative abundance (in decimal form).
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Sum the Results: Add up all the products calculated in the previous step. The result is the weighted atomic mass of the element.
Formula
The formula for calculating weighted atomic mass is:
Weighted Atomic Mass = (Mass of Isotope 1 × Abundance of Isotope 1) + (Mass of Isotope 2 × Abundance of Isotope 2) + ...
Example
Let's say we have an element with two isotopes:
- Isotope A: Mass = 20 amu, Abundance = 80% (0.80)
- Isotope B: Mass = 22 amu, Abundance = 20% (0.20)
The weighted atomic mass would be:
(20 amu × 0.80) + (22 amu × 0.20) = 16 amu + 4.4 amu = 20.4 amu
Therefore, the weighted atomic mass of this element is 20.4 amu.
Importance
The weighted atomic mass is important because it reflects the average mass of an element taking into account the different isotopes and their proportions in nature. It is the value listed on most periodic tables and is used in various chemical calculations.
