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Average Atomic Mass Calculation and Introduction to Moles

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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Average Atomic Mass Calculation and Introduction to Moles

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  1. Homework 4 due today:14 Feb Homework 5 posted-due Wednesday 21 Feb

  2. 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

  3. 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

  4. 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

  5. Need more, or are you exhausted ???

  6. In- class example with mercury: Computing the average mass of Hg from abundance data 1881 Sum= 20057 3144 5720 2492 5555 1246

  7. AV. MASS =%P1*m1+%P2*m2 +… 100 • Average mass of Hg =20057 =200.57 100

  8. Chapter 3: MOLES- Every science involves counting Astronomers count …… Animalbiologistscount … Chemists count……. Bacteriologistscount ...

  9. Astronomers can see…. Animalbiologistscan see … Chemists can’t see…. Bacteriologists can see… molecules

  10. 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 ??

  11. 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)

  12. 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 ???

  13. Chemists count atoms and molecules using… Moles

  14. 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.

  15. 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… 

  16. A better beginning: the mole concept is really … the same idea as a `dozen.’ 

  17. IN-CLASS EGG CALCULATIONS

  18. EGG WORLD CHEMISTRY WORLD Smallest unit 1 egg 1 molecule Practical Counting unit Mole (The chemist’s dozen) Dozen

  19. 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)

  20. 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

  21. 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)

  22. 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

  23. In – class exercise on Molecular Weight (MW) calculations Find the MW of… H2SO4 2*1 + 1*32 + 4*16 = 98 g/mol

  24. In – class exercise on Molecular Weight (MW) calculations (continued) Find the MW of… CaCl2 1*40 + 2*35.4 = 110.4 g/mol

  25. 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

  26. 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

  27. IN-CLASS simple mole calculations (on board) the dozen method way

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