Calculating Average Atomic Mass using Weighted Average, Slides of Chemistry

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How to Find

Average Atomic Mass

Copy when you see…

Do you know what a weighted average is?

• Your grades are calculated by weighted average:
  • 40% minor
  • 60% major
• When you want to calculate your class average,

this is how you do it!

Since isotopes are atoms of the same element with different masses:

• We use a weighted average to calculate the

average atomic mass because some isotopes

are more common than others.

• We use this formula:

% abundance x mass (for each isotope)

THEN add them all together!

REMEMBER, change the % to a decimal!!

Example:

• Chlorine-35 ( Cl) makes up 76% of chlorine atoms and chlorine-37 ( Cl) makes up 24%. What is the average atomic mass for chlorine? (% abundance x mass)

0.76 x 35 =

0.24 x 37 =

Then, add them up!

35.48 amu ← This is the

average atomic mass for chlorine! 35 17 37 17