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This text provides examples of calculating average atomic masses of elements and introduces the concept of moles in chemistry. It also includes in-class exercises on molecular weight calculations and simple mole calculations.
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Homework 4 due today:14 Feb Homework 5 posted-due Wednesday 21 Feb
estimate average atomic masses for elements below Isotope ~mass % abundance ~ average mass 1H 1.008 99.985 2H 2 .014 0.015 1.008 28Si 27.98 92.2 29Si 28.98 4.8 30Si 29.97 3.0 28.081 35Cl 34.97 75.8 37Cl 36.97 24.2 35.454
A new, synthetic element, X, has two isotopes. The lighter isotope weighs 250 amu and is 45% of the total. The heaver isotope weights 260 amu and 55% of the total. What is the average mass of X ? • 255 amu • 2555 amu • 254.4 amu • 255.5 amu
An element has three isotopes with the following masses and % abundances: (100 amu ,0.1%), (105 amu, 5%) and (110 amu 94.9 %). Without doing a calculation which average mass below makes the most sense ? • 105 amu • 107.9 amu • 109.7 amu • 110 amu
In- class example with mercury: Computing the average mass of Hg from abundance data 1881 Sum= 20057 3144 5720 2492 5555 1246
AV. MASS =%P1*m1+%P2*m2 +… 100 • Average mass of Hg =20057 =200.57 100
Chapter 3: MOLES- Every science involves counting Astronomers count …… Animalbiologistscount … Chemists count……. Bacteriologistscount ...
Astronomers can see…. Animalbiologistscan see … Chemists can’t see…. Bacteriologists can see… molecules
Even worse news…even if we could see molecules…guess long would it take to count the number of water molecules in a tea cup ??
Let’s use America’s fastest computer (the Titan1) to count all the molecules of water in a teacup assuming it counted at its maximum processor rate =17,600,000,000,000,000 molecules/second ( 17.6 petaflops) How long would it take to count them all ? Answer: ~ 1.1 years 1 ~ 1 million times faster than a typical laptop (It houses 2.7 million processors in parallel)
From your lecture syllabus…. Student Learning Outcome 5 Students should be able to perform basic chemical calculations connected to: • mole-weight-count conversions • reaction stoichiometry • limiting yields. Translation: How do chemists count atoms and molecules without actually having to count them ???
A trip to Mole land Moles connect the atomic world to human world ~ 1027 molecules in the shape of a couch potato ~0.00000002m ~2m ~30 CO2 molecules arranged in shape of muffin man (IBM) • Moles allow us to conveniently connect between these worlds using a scale.
The mole: starting definitions What is a chemical mole ? The classic textbook rant…….(but not your text’s rant, fortunately) • # atoms in 12.000 g of 12C • 6.0221335 *1023 atoms (Avogadro’s Number) …Not that helpful when first starting out…
A better beginning: the mole concept is really … the same idea as a `dozen.’
EGG WORLD CHEMISTRY WORLD Smallest unit 1 egg 1 molecule Practical Counting unit Mole (The chemist’s dozen) Dozen
Eggs vs. chemistry world (continued) EGG WORLD CHEMISTRY WORLD # units in practical counting unit 1 mole 6.022 *1023 1 dozen=12 (Avogadro’s #) Practical mass unit grams/dozen (WEIGH) grams/mole =gram molecular weight = MW (molecular weight) (ADD)
THE ONE PLACE EGG WORLD AND CHEMISTRY WORLD DEVIATE A TEENY BIT: Egg world Chemistry world Mass dozen Mass = molecular weight mol (MW) Must weigh box of eggs Just add up atomic masses using Periodic Table in grams
Mass (grams) = MW=Molecular mol Weight = sum of masses in grams of component elements in a compound . (element masses are average masses taken from Periodic Table)
Example of MW Calculation: What is theMWof methane =CH4 ?? atomic mass (Table) # atoms/molecule product C H 12.011~12 1.00794~1 1 4 ~12*1=12 ~1*4 = 4 Sum = MW = 12+4=16 g/mole
In – class exercise on Molecular Weight (MW) calculations Find the MW of… H2SO4 2*1 + 1*32 + 4*16 = 98 g/mol
In – class exercise on Molecular Weight (MW) calculations (continued) Find the MW of… CaCl2 1*40 + 2*35.4 = 110.4 g/mol
In – class exercise on Molecular Weight (MW) calculations (continued) Find the MW of… Cu3(PO4)2 3*63.5 + 2*(1*31 +4*16) = 380.5 g/mol
In – class exercise on Molecular Weight (MW) calculations (continued) Find the MW of… CaCl2*2H2O 2 `waters of hydration’ per CaCl2 1*40 + 2*35.4 + 2*(2*1 + 1*16) =146.8 g/mol
IN-CLASS simple mole calculations (on board) the dozen method way