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Presentation on theme: "2.8-2.9 Atomic Mass & Molar Mass"— Presentation transcript:
2.8-2.9 Atomic Mass & Molar Mass
What is an elements atomic mass?
The weighted average of its naturally occurring isotopes
When looking at an element on the periodic table of elements, where is atomic mass found?
Usually the top left hand corner of each elements box. However, on the periodic table that you will use for your AP Exam(s), this number will be located below each elements chemical symbol.
How is it calculated? Show all work for the following problem:
Magnesium has three naturally occurring isotopes with masses 23.99
and natural abundances of 78.99%, 10.00% and 11.01% respectively. Calculate the atomic mass of magnesium.
= (23.99 X .7899) + (24.99 X .1000) + (25.98 X .1101) =
Atomic mass of Mg = 24.31
A technique that separates particles according to their mass.
Can be used to identify atoms that make up a molecule, and the percent abundances of isotopes of an element.
After a sample is ionized, and the ions travel through the mass spectrometer, they strike a detector that puts out signals, and ultimately produces a mass spectrum of the sample to be analyzed.
Mass Spectrum of Boron
Boron has an atomic mass of around 10.8
If you analyze this spectrum you can see that there are two lines, the larger at 11
. (m/z = mass charge ratio)
The two lines indicate that boron, which is monatomic, has two isotopes boron-10 and boron-11.
On the following page, we will use the spectrum to calculate boron’s atomic mass: