Chemistry SM-1131Week 8 Lesson 1

Dr. Jesse ReichAssistant Professor of Chemistry

Massachusetts Maritime AcademyFall 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 (1859-1936) When I was one-and-twentyI heard a wise man say,'Give crowns and pounds and guineasBut not your heart away;Give pearls away and rubiesBut keep your fancy free.'But I was one-and-twenty,No use to talk to me. When I was one-and-twentyI heard him say again,'The heart out of the bosomWas never given in vain;'Tis paid with sighs a plentyAnd 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 N2 is going to be twice the mass of 1 atom of N

• So, if N has an atomic mass of 14.01, then N2 must have a mass of 2x14.01 or 28.02

Molecular Mass

• Ozone has the formula O3, what is it’s molecular mass

• 1 Oxygen has a mass of 16.00• 3x(16.00) has a mass of 48.00 amu

More Complex Masses

• Water has the formula H2O

• The molecular mass is going to be from 2H atoms and 1 O atom, so

• 2x(1.0079) + 1x(16.00)= 18.0158amu

Sugar

• C6H12O6

• 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=

1.2044e24 atoms• 3 moles of atoms = 3(6.022e23)atoms=

1.8066e24

Moles

• 1 mole of kittens = 6.022e23 kittens• 2 moles of kittens= 2(6.022e23)kittens=

1.2044e24 kittens• 3 moles of kittens = 3(6.022e23)kittens=

1.8066e24

Why a 6.022e23• 1 amu = 1.66e-24 grams• So, 1.66e-24gx6.022e23= 0.99652g 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 N2 how much would it weigh?• Atomic Mass of N= 14.01 amu• Molecular Mass of N2= 28.02 amu• 6.022e23 atoms of N2 x 28.02 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

• 24.00 moles of He 6.022e23 atoms = 1.445e25 atoms

1 mole

Atoms into Moles

• You have 18.066 e23 atoms of Cu many many moles of Cu do you have?

18.066e23 atoms x 1 mol = 3.0000 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 H2O has how many moles of Oxygen in it?25g H2O x 1 mole H2O x 1 mole O = 1.4 moles

18 gram H2O 1 moles H2O

25 g H2O has how many moles of hydrogen in it?

25g H2O x 1 mole H2O x 2 mole H = 2.8 moles

18 gram H2O 1 moles H2O

Grams molecules grams of an element

• Grams of a molecule moles of a molecule moles of an element

25 g H2O has how many grams of Oxygen in it?25g H2O x 1 mole H2O x 1 mole O x 16 g O = 22 grams O

18 gram H2O 1 moles H2O 1 mole O

25 g H2O has how many grams of Oxygen in it?25g H2O x 1 mole H2O x 2 mole H x 1.0 g H = 2.8 grams H

18 gram H2O 1 moles H2O 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)Cl2 has how many grams of Fe(II) in it?

88g Fe(II)Cl2 x 1 mole Fe(II)Cl2 x 1 mole Fe(II) x 55.85 g Fe(II) = 39 grams Fe(II)

126.75 gram Fe(II)Cl2 1 moles Fe(II)Cl2 1 mole Fe(II)

88 g Fe(II)Cl2 has how many grams of Cl in it?

88g Fe(II)Cl2 x 1 mole Fe(II)Cl2 x 1 mole Cl x 35.45 g Cl = 25 grams Cl

126.75 g Fe(II)Cl2 1 moles Fe(II)Cl2 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

• CO2

1x(12) amu x 100 = 27% carbon 44 amu• SF6

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 gram2- Write the number of grams of element A3- Convert from grams to moles by multiplying by 1 mole per molecular mass

(in grams)

4- repeat steps 2 and 3 for every element5- divide all number of moles by the smallest number of moles6- 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 Na1Cl1

Iron(II) Carbonate

• In class right now. Figure out the formula (carbonate is (CO3)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 C2H4 and C4H8

• 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 C2H4 and C4H8

• 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 CH2O & C6H12O6, C2H2 & C6H6, or C2Cl2 & C6Cl6

• If you can’t do some of those question get help!