Homework 4 due today 14 Feb Homework 5

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

estimate average atomic masses for elements below Isotope ~mass 1 H 1. 008 2 H 2. 014 28 Si 29 Si 30 Si 35 Cl 37 Cl 27. 98 28. 98 29. 97 34. 97 36. 97 % abundance ~ average mass 99. 985 1. 008 0. 015 92. 2 4. 8 3. 0 75. 8 24. 2 28. 081 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 ? A. 255 amu B. 2555 amu C. 254. 4 amu D. 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 ? A. 105 amu B. 107. 9 amu C. 109. 7 amu D. 110 amu

Need more, or are you exhausted ? ? ?

In- class example with mercury: Computing the average mass of Hg from abundance data #p #n 80 Isotope %Pk mk *%Pk mass, mk 1881 118 198 9. 5 80 119 199 15. 8 3144 80 120 200 28. 6 5720 80 121 201 12. 4 80 122 202 27. 5 2492 5555 80 124 204 6. 2 1246 Sum= 20057

AV. MASS =%P 1*m 1+%P 2*m 2 +… 100 ÞAverage mass of Hg =20057 =200. 57 100

Chapter 3: MOLES- Every science involves counting Astronomers count …… Animal biologists count … Bacteriologists count. . . Chemists count…….

Astronomers can see…. Animal biologists can see … Bacteriologists can see… Chemists can’t 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 Titan 1) to count all the molecules of water in a teacup assuming it counted at its maximum processor rate = 17, 600, 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 ? ? ?

Chemists count atoms and molecules using… Moles

A trip to Mole land Moles connect the atomic world to human world ~ 1027 molecules in the shape of a couch potato ~0. 00000002 m ~30 CO 2 ~2 m ØMoles allow us to molecules conveniently connect between arranged in shape of muffin these worlds using a scale. man (IBM)

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 12 C 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. ’

IN-CLASS EGG CALCULATIONS

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

Eggs vs. chemistry world (continued) EGG WORLD # units in practical 1 dozen=12 counting unit Practical grams/dozen mass unit (WEIGH) CHEMISTRY WORLD 1 mole 6. 022 *1023 (Avogadro’s #) grams/mole =gram molecular weight = MW (molecular weight) (ADD)

THE ONE PLACE EGG WORLD AND CHEMISTRY WORLD DEVIATE A TEENY BIT: Egg world Mass dozen Must weigh box of eggs Chemistry world Mass = molecular weight mol (MW) 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 the MW of methane =CH 4 ? ? atomic mass (Table) # atoms/molecule C 12. 011~12 H 1. 00794~1 1 4 product ~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… H 2 SO 4 2*1 + 1*32 + 4*16 = 98 g/mol

In – class exercise on Molecular Weight (MW) calculations (continued) Find the MW of… Ca. Cl 2 1*40 + 2*35. 4 = 110. 4 g/mol

In – class exercise on Molecular Weight (MW) calculations (continued) Find the MW of… Cu 3(PO 4)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… Ca. Cl 2*2 H 2 O 2 `waters of hydration’ per Ca. Cl 2 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