Amu Chemistry

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Department of Chemistry TAMU 3255 College Station, TX. Fe: 1 x 55.85 = 55.85 amu. Cl: 1 x 35.45 = +106.35 amu Total = 162.20 amu. The formula mass of FeCl 3 is 162.2 amu. When we distribute the subscript 3 through the parentheses containing the formula for the ammonium ion, we see that we have 3 nitrogen atoms and 12 hydrogen atoms. Thus, we set up the sum as follows: N: 3 x 14.00 = 42.00 amu. In chemistry, one often needs to calculate different forms of measurement. In this example, we calculate atomic abundance from atomic mass. The element boron consists of two isotopes, 10 5 B and 11 5 B.

Amu Chemistry

Method of Assessment

There will be two (2) tests in the course, a midterm and a final given in modules four and eight respectively. The tests in this class will consist of multiple choice questions and/or short answer problems. Tests will be three hours long and focus on the material covered in the course. Tests may be administered using the Examity test proctoring service. Please verify in the course announcements and/or the Lessons tab if the course will use test proctoring. It is the student’s responsibility to notify the instructor and the test proctoring service regarding any schedule changes or non- disability related accommodations. The course will have a comprehensive paper that will measure student understanding of the course objectives along with researhc and writting skills. Please refer to the APA Manual 6th edition for the format of the paper.


Course assignments will be given using multimedia software (e.g. MyLabsPlus, Connect, etc.). These assignments will be a series of quizzes consisting of exercises, problems, and simulations. Assignments will have a specific due date with specific instructions. Late assignments will be subject to the university’s Late Work/Make-up Policy detailed in the student handbook. Please be advised the instructor reserves the right to implement their own late assignment policy.


Throughout the term, homework will be given in several modules to test student understanding of the material. Homework will be given using multimedia software (e.g. MyLabsPlus, Connect, etc.) and consist of questions, problems, or simlations. Please be advised the instructor reserves the right to implement their own late assignment policy.

D.Discussion Forums:

Participation in the discussion forums is an essential component of the final grade. All students are expected to engage in lively discussions and answer instructor follow-up questions. The quality of participations along with student netiquette will be a part of the grade.

Assessment of the Course Objectives

Course Objective(s)

Assessment Method(s)

1 - 6

Test question, paper, assignment, discussion, and homework

The following distribution will be used in assigning grades (decimal points will be rounded to the nearest whole number at semester’s end).


Quality Points/Grading Percent


4.0/ 100 – 94


3.67/ 93 – 90


3.33/ 89 – 87


3.0/ 86 – 84


2.67/ 83 –80


2.33/ 79 – 77


2.0/ 76 – 73


1.67/ 72 – 70


1.33/ 69 – 67


1.0/ 66 – 64


0.67/ 63 – 60


0.0/ 59 – 0

NameGrade %
Discussion Forums10.00 %
Week 4 Forum5.00 %
Week 8 Forum5.00 %
Homework20.00 %
Chapter One Homework3.33 %
Chapter Two Homework3.33 %
Chapter Three Homework3.33 %
Chapter Four Homework3.33 %
Chapter Five Homework3.33 %
Chapter Six Homework3.33 %
Quizzes30.00 %
Chapter One Quiz5.00 %
Chapter Two Quiz5.00 %
Chapter Three Quiz5.00 %
Chapter Four Quiz5.00 %
Chapter Five Quiz5.00 %
Chapter Six Quiz5.00 %
Final Assessment10.00 %
Week 8 Paper10.00 %
Tests30.00 %
Midterm15.00 %
Final15.00 %

How do you calculate atomic mass?

To calculate the atomic mass of an element, we have to calculate how much each isotope contributes to the mass of the atom. To accomplish this, we usually use an approach called the weighted average. The weighted average takes into account the mass and percentage abundance of each isotope.

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What’s percentage abundance?

It is the proportion of atoms of an isotope in a sample of an element taken from the natural world. Percentage abundance is always reported as a percentage, and it is calculated as: (number of atoms of an isotope) divided by (the total number of atoms of all isotopes of that element) multiplied by 100. Percentage abundance usually can be divided by 100 to get fractional abundance.

How do you use weighted average to calculate atomic mass?

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To use weighted average, we must take into account the mass and percentage abundance of each isotope. Let’s use the data in the following table to show how weighted average is used to calculate the atomic mass of oxygen.


To calculate the atomic mass of oxygen using the data in the above table, we must first

  • multiply the mass of each isotope by its corresponding natural abundance (percentage abundance). But, since the abundance is in %, you must also divide each abundance value by 100.

And second,

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  • Sum the result to get the atomic mass of the element


Atomic mass of oxygen = 15.995 amu (99.76/100) + 16.999 amu (.04/100) + 17.999 amu (.2/100)

= 15.956612 amu + 0.0067996 amu + 0.035998 amu

= 15.9994096 amu

= 16.00 amu

Note that the abundance in percent always add up to 100 %. Meaning if the previous question had left out the percentage abundance value for oxygen-18, you could have gotten it by subtracting the sum of the percentage abundance for oxygen-16 and oxygen-17 from 100% to get the percentage abundance for oxygen-18.

That is: 100% – (99.76% + .04%) = 0.2% for oxygen-18

Generally, you can apply this approach to figure out missing percentage abundance when you know the percentage abundance values for all, but one isotope.

If you examine the atomic masses on the periodic table, you will notice that they are fractional. Why are they fractional? They are fractional because atoms exist as isotopes. And isotopes do not have the same mass and abundance in nature. As a result, to calculate the atomic mass of an element, we have to calculate how much each isotope contributes to the mass of the atom.

Why are there no units of grams attached to the atomic mass of oxygen?

There are no units attached to atomic masses because atomic masses are relative atomic masses. Relative in this sense means one thing is compared to another. So, relative atomic mass means the mass of one atom is compared to the mass of another atom.

The atom to which other atoms are compared to is usually called the standard. At present, an isotope of carbon called carbon-12 (C-12) is selected as the standard and assigned an atomic mass of exactly 12 amu, where amu stands for atomic mass units. Therefore, the mass of every other atom on the periodic table is determined by how light or heavy it is when compared to the mass of C-12.

What instrument is used to measure the relative masses of atoms?

At present, mass spectrometry is the technique used to measure the relative masses of atoms and their percentage abundance in nature. In a mass spectrometer, atoms interact with a magnetic field and separate according to their mass to charge ratio. As they separate according to this ratio, their percentage abundance and relative atomic masses can be calculated. Let’s use the following example to illustrate how the relative mass of an atom is calculated using carbon-12 as the standard.


If a chemist measured a sample in a mass spectrometer and determined that the mass ratio of 28Si (silicon-28) to 12C (carbon-12) is: 2.33, calculate the mass of 28Si?


Since the mass of carbon-12 is is assigned a value of 12, then it follows that the mass of 28Si = mass ratio of (28Si/12C) multiplied by mass of 12C

Thus, mass of 28Si = 2.33 x 12 amu = 27.98 amu

To learn more about atomic mass and Avogadro’s number, click here.