2 Atomic MassWhat 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 amu, amu and 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 = amu
3 Mass SpectrometryA 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.
4 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:
5 Mass Spectrum of Boron (Continued) First, we need to calculate relative abundance:Abundance of B-10 = 25.0% / (25.0% %) = 20.0%Abundance of B-11 = 100.0% / (25.0% %) = 80.0%Next, let’s calculate atomic mass:B atomic mass = (.2000 X 10) + (.8000 X 11) = 10.8 amu
6 Avogadro’s NumberWhat 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.
7 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 x1023 atoms Ag=X mol AgX = 4.65x10-2 mol Ag
8 Molar Mass and mass to mole conversions Molar mass g/mol = Atomic mass in amuExample: carbons molar mass = g/molcarbon’s atomic mass = amu12.01 g carbon = 1 mol carbon = 6.022X1023 atoms carbonCalculate the number of moles of copper in 35.8 g pure copper.35.8 g Cu g Cu= X = 5.63X10-1 mol CuX mol Cu
9 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 X1023 atoms W= X = 3.74X10-2 mol WX mol WNext, convert moles to atoms3.74X10-2 mol W mol W= X = 6.87 g WX g W
10 Pgs #’s 76, 80, 82, and 91.Study for Chapter 2 Quiz