This presentation is the property of its rightful owner.
1 / 12

Calculations in Chemistry: PowerPoint PPT Presentation

Calculations in Chemistry:. Average Atomic Mass. Isotopes. An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass as a result of a different number of neutrons.

Calculations in Chemistry:

Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.

- - - - - - - - - - - - - - - - - - - - - - - - - - E N D - - - - - - - - - - - - - - - - - - - - - - - - - -

Calculations in Chemistry:

Average Atomic Mass

Isotopes

• An element can exist in a number of forms, called isotopes. Isotopes are forms of the same atom that vary in mass as a result of a different number of neutrons.

• For example, there are two different types (isotopes) of copper atoms. One type of copper atoms weighs in at 62.93 amu, the other has a mass of 64.94 amu.

• These two isotopes have different proportions in a natural sample of copper.

• The lighter isotope is more common with  69.09% of the naturally occurring copper having a mass of 62.93 amu per atom.

• What is the percent abundance of the other isotope?

• The remainder of the atoms, 30.91 %, have a mass of 64.94 amu.

Average Atomic Mass

• To find the AVERAGE ATOMIC MASS of an atom, we take into account all of the isotopes that exist and the percentage of each type.

•  The calculation of the average atomic mass is a WEIGHTED AVERAGE.

Calculation of Average Mass

Average atomic mass =

Σ (mass of isotope × relative abundance)

Σ = sum or to add them all together

Calculation of Average Mass

• The bottom line is that to find the average atomic mass of copper, we insert the information about copper’s isotopes into the formula and solve.

• There are two isotopes, so we will be adding the contributions of 2 isotopes. (That’s where the Σ sign comes in. )

• The relative abundance is simply the percentage of the isotope, but in decimal format.  69.09% corresponds to a relative abundance of 0.6909.

Calculation of Average Mass

Average atomic mass of copper =

(62.93 amu × 0.6909)

[the mass and abundance of isotope #1]

+

(64.94 amu × 0.3091)

[the mass and abundance of isotope #2]

=

63.55

Calculation of Average Mass

• From the calculation, we know that an AVERAGE atom of copper has a mass of 63.55 amu.

• Notice that in this problem, we would predict that the average is closer to the weight of the lighter isotope.

• This is because the lighter form of copper is more abundant.

Practice

• Silver = 107.87 amu

• Silicon = 28.09 amu

• Iron = 55.84 amu

Homework

• Read section 1.4 in the text to support what we did in class today.

• pg27 #1

• Pg 29 #1-5,9