2022-10-15

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Chemistry in a Nutshell

Learn how to calculate the average atomic mass of magnesium based on percent abundances and the mass of each isotope. This 3-step process is the same for any element.

The Problem:

A sample of magnesium is found to contain 78.70% Mg-24 atoms (mass 23.98 amu), 10.13% Mg-25 atoms (mass 24.99 amu), and 11.17% Mg-26 atoms (mass 25.98 amu). Calculate the average mass of a Mg atom.

Free, legal open source chemistry textbook: https://openstax.org/details/books/chemistry-2e

This problem is Example 2.4 part B, page 86.

Periodic Table is from https://sciencenotes.org/printable-periodic-table/

Time Stamps:

0:00 Intro

0:42 Problem

1:18 Weighted Average

1:45 Step 1: Percent Abundances to Decimals

2:09 Step 2: Multiply by Masses

2:36 Step 3: Find the Sum

2:46 Don't Rely on Periodic Table Mass! I can still test you on this type of question!

3:58 Evaluating Significant Figures for This Problem

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