2.8-2.9 Atomic Mass & Molar Mass

Atomic 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 amu, 24.99 amu and 25.98 amu and natural abundances of 78.99%, 10.00% and 11.01% respectively. Calculate the atomic mass of magnesium. Mgiso1 + Mgiso2+ Mgiso3 = (23.99 X .7899) + (24.99 X .1000) + (25.98 X .1101) = Atomic mass of Mg = 24.31 amu

Mass Spectrometry 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 amu If you analyze this spectrum you can see that there are two lines, the larger at 11 amu. (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:

Mass Spectrum of Boron (Continued) First, we need to calculate relative abundance: Abundance of B-10 = 25.0% / (25.0% + 100.0%) = 20.0% Abundance of B-11 = 100.0% / (25.0% + 100.0%) = 80.0% Next, let’s calculate atomic mass: B atomic mass = (.2000 X 10) + (.8000 X 11) = 10.8 amu

Avogadro’s Number What is Avogadro’s number, and how does it get its specific value? 6.022x1023 mol- (You don’t need to memorize this number) is equal to the amount of atoms in 12.0 g of pure carbon 12.

Mole Conversions (atoms to moles) A pure silver ring contains 2.80X1022 silver atoms. How many moles of silver atoms does it contain? 2.80X1022 atoms Ag 6.022x1023 atoms Ag ------------------------------ = ------------------------------ X 1.00 mol Ag X = 4.65x10-2 mol Ag

Molar Mass and mass to mole conversions Molar mass g/mol = Atomic mass in amu Example: carbons molar mass = 12.01 g/mol carbon’s atomic mass = 12.01 amu 12.01 g carbon = 1 mol carbon = 6.022X1023 atoms carbon Calculate the number of moles of copper in 35.8 g pure copper. 35.8 g Cu 63.55 g Cu --------------- = -------------------- X = 5.63X10-1 mol Cu X 1.00 mol Cu

Mole Conversions (atoms to mass) Calculate the mass of 2.25X1022 tungsten atoms. This is a two step problem: First, convert atoms to moles: 2.25X1022 atoms W 6.022X1023 atoms W ---------------------------------------- = ------------------------------------------------ X = 3.74X10-2 mol W X 1.00 mol W Next, convert moles to atoms 3.74X10-2 mol W 1.00 mol W ---------------------------------------- = ------------------------------------------------ X = 6.87 g W X 183.8 g W

Pgs. 81-82 #’s 76, 80, 82, and 91. Study for Chapter 2 Quiz