Chemistry SM-1131 Week 8 Lesson 1 Dr. Jesse Reich Assistant Professor of Chemistry Massachusetts Maritime Academy Fall 2008

Class Today Poem Quiz Correction Grams, atoms, mols, avogadro’s number “Moles,” g-> molecules, molecules  atoms, grams molecule  gram element, mass percent, mass percent from a chemical formula Take Worksheets on moles Wednesday (each completed packet is worth 5 test points) Test Friday: Nomenclature and formula writing (Ionic, molecular, acids) mole and gram conversions.

When I was one-and- twenty By A. E. Housman ( )A. E. Housman When I was one-and-twenty I heard a wise man say, 'Give crowns and pounds and guineas But not your heart away; Give pearls away and rubies But keep your fancy free.' But I was one-and-twenty, No use to talk to me. When I was one-and-twenty I heard him say again, 'The heart out of the bosom Was never given in vain; 'Tis paid with sighs a plenty And sold for endless rue.' And I am two-and-twenty, And oh, 'tis true, 'tis true.

Quiz Correction Put away your pens Take out your quizes Switch quizes with your neighbor Print your name in red ink on their quiz Put a big X by a wrong answer Put a check by a correct one Grade your neighbors quiz: 8-0.1(wrong answers)= quiz grade

Molecular Mass To figure out molecular mass you have to know the atomic mass. Let’s start easily Ne- it exists by itself and doesn’t form molecules. It’s mass is just the atomic mass of Ne, which is 20.18

Simple Molecule Mass The mass of N 2 is going to be twice the mass of 1 atom of N So, if N has an atomic mass of 14.01, then N 2 must have a mass of 2x14.01 or 28.02

Molecular Mass Ozone has the formula O 3, what is it’s molecular mass 1 Oxygen has a mass of x(16.00) has a mass of amu

More Complex Masses Water has the formula H 2 O The molecular mass is going to be from 2H atoms and 1 O atom, so 2x(1.0079) + 1x(16.00)= amu

Sugar C 6 H 12 O 6 6(12.01) + 12(1.0079) + 6(16.00)= ???

Moles Dozen: 12 somethings Baker’s Dozen: 13 Somethings A Score: 20 Somethings Avogadro’s number: 1 mol= 6.022e23 somethings

See how it works A dozen atoms = 12 atoms A baker’s dozen atoms = 13 atoms A score of atoms = 20 atoms A mole of atoms = 6.022e23 atoms

Moles It just means a big number. 602,200,000,000,000,000,000,000,000 But we do this because it converts amu to grams

Moles 1 mole of atoms = 6.022e23 atoms 2 moles of atoms= 2(6.022e23)atoms= e24 atoms 3 moles of atoms = 3(6.022e23)atoms= e24

Moles 1 mole of kittens = 6.022e23 kittens 2 moles of kittens= 2(6.022e23)kittens= e24 kittens 3 moles of kittens = 3(6.022e23)kittens= e24

Why a 6.022e23 1 amu = 1.66e-24 grams So, 1.66e-24gx6.022e23= g which is very similar to 1g. The point is that if you multiply the mass of something in amu you can convert amu into a mass in grams We don’t weigh anything in amu, but we do in grams so this is useful. So, if we multiply the atomic mass of something by 1 mol it turns from amu into grams.

If you have 1 mole of N 2 how much would it weigh? Atomic Mass of N= amu Molecular Mass of N 2 = amu 6.022e23 atoms of N 2 x amu x 1.66e-24g = 1 atom 1 amu Which equals 28.02g. So, 1 mol x molecular mass = # of grams

What do we do with this? Chemists generally convert moles into atoms. Atoms into Moles Moles into grams Grams into Moles

Moles into atoms 1 mole has 6.022e23 atoms in it 5 moles of Ne x 6.022e23 atoms = 3.011e24 atoms 1 mole moles of He 6.022e23 atoms = 1.445e25 atoms 1 mole

Atoms into Moles You have e23 atoms of Cu many many moles of Cu do you have? e23 atoms x 1 mol = mol 6.022e23 atoms

So Atoms x 1 mole = moles 6.022e23 atoms Moles x 6.022e23 atoms = atoms 1 mole

Moles to grams We also convert moles into grams You can’t weigh a mole, you weigh a gram Moles x molecular mass in grams = grams 1 mole

Grams to Moles Grams -> Moles Xgrams x moles = moles Atomic mass

New Material “Moles,” grams molecule  gram element, mass percent, mass percent from a chemical formula

Mole 1 mole of something means 6.022e23 somethings Remember 1 mole of a compound = the molecular mass in grams

Grams molecules  moles of an element Grams of a molecule  moles of a molecule  moles of an element 25 g H 2 O has how many moles of Oxygen in it? 25g H 2 O x 1 mole H 2 O x 1 mole O = 1.4 moles 18 gram H 2 O 1 moles H 2 O 25 g H 2 O has how many moles of hydrogen in it? 25g H 2 O x 1 mole H 2 O x 2 mole H = 2.8 moles 18 gram H 2 O 1 moles H 2 O

Grams molecules  grams of an element Grams of a molecule  moles of a molecule  moles of an element 25 g H 2 O has how many grams of Oxygen in it? 25g H 2 O x 1 mole H 2 O x 1 mole O x 16 g O = 22 grams O 18 gram H 2 O 1 moles H 2 O 1 mole O 25 g H 2 O has how many grams of Oxygen in it? 25g H 2 O x 1 mole H 2 O x 2 mole H x 1.0 g H = 2.8 grams H 18 gram H 2 O 1 moles H 2 O 1 mole O

Grams molecules  grams of an element Grams of a molecule  moles of a molecule  moles of an element 88 g Fe(II)Cl 2 has how many grams of Fe(II) in it? 88g Fe(II)Cl 2 x 1 mole Fe(II)Cl 2 x 1 mole Fe(II) x g Fe(II) = 39 grams Fe(II) gram Fe(II)Cl 2 1 moles Fe(II)Cl 2 1 mole Fe(II) 88 g Fe(II)Cl 2 has how many grams of Cl in it? 88g Fe(II)Cl 2 x 1 mole Fe(II)Cl 2 x 1 mole Cl x g Cl = 25 grams Cl g Fe(II)Cl 2 1 moles Fe(II)Cl 2 1 mole Fe(II)

Mass Percent Mass percent describes how much of a compound 1 element makes up A high mass percent means it makes up most of the mass of the compound. A low mass percent means it makes up just a little mass of the compound. Whenever we talk about a percent we mean Little Number x 100 = percent Big Number

Mass percent For Element A of compound AB Atomic Mass A x # of A atoms x 100 = mass percent Molecular mass of AB CO 2 1x(12) amu x 100 = 27% carbon 44 amu SF 6 100xMass of 6 F atoms or 114 amu x 100=88% Molecular Mass 146 amu

Percent Composition Percent Composition is when you figure out the mass percent for every element in the compound. Sodium chloride has the formula NaCl. What is the percent composition of all elements? 23 amu / 58 amu x 100 = 39 % Na 35 amu / 58 amu x 100 = 61 % Cl

You can do this backwards too Racecar is a palandrome, but it’s harder going backwards Cl makes up 61% of sodium chloride. What’s the formula? 1- convert % to gram 2- Write the number of grams of element A 3- Convert from grams to moles by multiplying by 1 mole per molecular mass (in grams) 4- repeat steps 2 and 3 for every element 5- divide all number of moles by the smallest number of moles 6- reduce, double, or triple until all atoms are expressed as whole numbers, then write the formula

Go through the steps 1- 61% Cl  61g Cl 2- 61g Cl 3- 61g Cl x 1 mole Cl = 1.7 moles Cl 35 g Cl 4- 39% Na  39g Na 5- 39g Na x 1 mole Na = 1.7 moles Na Decide which number is smaller. Divide both numbers by 1.7 moles. You get 1 mole of Na and Cl. So the formula must be Na 1 Cl 1

Iron(II) Carbonate In class right now. Figure out the formula (carbonate is (CO 3 ) 2- ). Figure out the percent composition of every element. Use the percent composition to back out the formula.

Empirical Formula What you are actually finding is the empirical formula. It’s the “reduced” form of a molecular formula. For ionic compounds the “reduced” form is always the molecular formula. For molecular compounds they are usually different.

Ethylene and butadiene C 2 H 4 and C 4 H 8 Perform a percent composition for both formulas Compute the empirical formula for both How are they related? Is the empirical formula the same thing as either molecular formula? How would the molecular formula of either relate to the empirical formula?

Emipirical Formula to molecular formula Molecular formula = n x empirical formula Molecular mass = n x empirical formula mass Molecular mass/empirical formula mass = n In a typical quiz/test problem you’ll usually have to calculate molecular mass from a formula, and you’ll be given either n or empirical formula mass and have to find the last variable.

Ethylene and butadiene C 2 H 4 and C 4 H 8 Calculate the n term for both of the above compounds.

Wednesday Wednesday is a working class day Come to class, be prepared with the moles packets, your book, a syllabus for homework problems, and a calculator I may or may not be here on Wednesday. You are expected to be in class. One of you will take attendance if I’m not here. Anyone missing class without sending me a note prior to class will be docked a letter grade from their final grade (B+  B).

Test Friday You will have a test Friday There will be formula writing questions There will be nomenclature questions There will be ionic vs acid vs molecular qs There will be mol -> gram, gram -> mol, gram  mol  atom, gram  mol  element  gram I will ask you for percent composition, empirical formula, molecular formula for either CH 2 O & C 6 H 12 O 6, C 2 H 2 & C 6 H 6, or C 2 Cl 2 & C 6 Cl 6 If you can’t do some of those question get help!