The Mole Concept

How many particles are present? In chemistry, it is important to know the # of atoms (or molecules) that you have. In chemistry, it is important to know the # of atoms (or molecules) that you have. Chemical formulas, like H 2 O, give us ratios of atoms: 2 H atoms bonded to 1 O atom. Chemical formulas, like H 2 O, give us ratios of atoms: 2 H atoms bonded to 1 O atom. Chemical equations: 2 H 2 + O 2  2 H 2 O give us ratios of reacting particles. Chemical equations: 2 H 2 + O 2  2 H 2 O give us ratios of reacting particles. Both formulas and equations are needed to calculate recipes for reactions. Both formulas and equations are needed to calculate recipes for reactions.

BIG Problem Atoms too small to see with the naked eye! Atoms too small to see with the naked eye! STM image of Si on 0.09 nm scale. (0.09 x m) (0.09 x m) = 9 x m = 9 x m Counting would take a lifetime! Counting would take a lifetime! Work HW 5-1 #1, p. 102

COUNTING BY WEIGHING How could you determine the number of marbles in the bag using a scale? Bag of marbles Bag of marbles Scale Scale How can you account for small variations in the masses of individual marbles?

Determining the Average Mass of a Marble (or an atom) Marbles (like atoms) will have similar but not necessarily identical masses. Marbles (like atoms) will have similar but not necessarily identical masses. Recall mass on periodic table represent average mass taking into account all of the isotopes of an atom. Recall mass on periodic table represent average mass taking into account all of the isotopes of an atom. Determination average mass of 10 marbles: Determination average mass of 10 marbles: ÷ 10 = 5.00 g

Counting by Weighing Determine total mass of all marbles in the bag: g Determine total mass of all marbles in the bag: g Let N = number of marbles Let N = number of marbles Average mass of 1 marble x N = total mass of all marbles Average mass of 1 marble x N = total mass of all marbles N = total mass ÷ average mass N = total mass ÷ average mass 2000 / 5.00 = 400. marbles 2000 / 5.00 = 400. marbles Work HW 5-1 #2, p x amu/ 10.8 amu/atom= 1 x 10 9

Could you go to the store and ask for 8,563 Rice Krispies please? NO! THEY SELL RICE KRISPIES BY THE BOX BECAUSE BOXES ARE EASIER TO COUNT!

Relationship between # of particles,“packing boxes” and mass If eachHas a mass of 10 mass units, (Ignore mass of “packing box”) 12 particles 1 “PACKING BOX” ? MASS UNITS WEIGHES ? MASS UNITS Ignoring the container mass, how much would 1 box weigh? ANS: 120 mass units (12 particles per box x 10 mass units per particles)

Relationship between # of particles,“packing boxes” and mass If you had a mass of 360, how many boxes would you have? (Recall 1 box weighs 120)

Relationship between # of particles,“packing boxes” and mass If you had a mass of 360, how many boxes would you have? (Recall 1 box weighs 120) Ans: 3 boxes; (360 mass units/ 120 mass units)

Relationship between # of particles,“packing boxes” and mass If you had a mass of 360, how many boxes would you have? (Recall 1 box weighs 120) Ans: 3 boxes; (360 mass units/ 120 mass units) How many particles are present? Ans: 36; (3 boxes x 12 particles per box) Work HW 5-1 #3, p A) 120 mass units B) 5 boxes C) 126 atoms; 1260 mass units

Examples of counting by grouping

Chemist’s Solution to Problem Counting of Atoms or Molecules The Mole Concept The mole connects the # of particles present with a property we can measure in the lab: mass. (Counting by grouping and counting by weighing) Can’t see Can measure

Definition of a Mole The number of atoms present in a g sample of Carbon x atoms

The Mole is a Number The mole represents a # of particles. The mole represents a # of particles. A pair = 2A dozen = 12 A pair = 2A dozen = 12 A mole = x A mole = x This number is often referred This number is often referred to as “Avogadro’s Number” Work HW 5-1 #4, p

The Mole is a BIG number x represents an incomprehensively large number! x represents an incomprehensively large number! If a mole of marbles were laid on the surface of the earth, how much of the earth would be covered? If a mole of marbles were laid on the surface of the earth, how much of the earth would be covered? Cover entire earth to a height of 50 miles! Cover entire earth to a height of 50 miles!

Developing the Definition of a Mole The definition of a mole was chosen to make it easy to determine the mass of a mole of atoms using the periodic table. The definition of a mole was chosen to make it easy to determine the mass of a mole of atoms using the periodic table.

Developing the Definition of a Mole Masses on periodic table represent the mass on 1 atom measured in amu. Masses on periodic table represent the mass on 1 atom measured in amu. 1 atom of C-12 = amu 1 atom of C-12 = amu In the lab we have to work with very large numbers of atoms by measuring masses in grams. In the lab we have to work with very large numbers of atoms by measuring masses in grams. 1 amu = 1.66 x g 1 amu = 1.66 x g Mass of 1 carbon atom = Mass of 1 carbon atom = amu x 1.66 x g amu x 1.66 x g = 1 amu 1 amu 1.99 x g

Where does the number of atoms in a mole come from? Mass of 1 C atom x # of C atoms = total mass of group of atoms Mass of 1 C atom x # of C atoms = total mass of group of atoms If you take the mass for carbon on the periodic table (12.00) measured in units of grams how many atoms would you have? If you take the mass for carbon on the periodic table (12.00) measured in units of grams how many atoms would you have? Mass of 1 C atom in grams = 1.99 x g Mass of 1 C atom in grams = 1.99 x g Let N = # of C atoms Let N = # of C atoms 1.99 x g x N = g 1.99 x g x N = g N = g/ 1.99 x g = x 10 23

What is the pattern in the following data? Element (atomic #) # of atoms present # of moles present Mass in grams He (#2)6.022 x g Li (#3)6.022 x g B (#5)6.022 x g C (#12)6.022 x g Cu (#29)6.022 x g Au (#79)6.022 x g

Understanding the Mole Concept Fill in the missing information Fill in the missing information Mass of 1 mole ____________________ of an element = atomic _________ in _______ (6.022 x atoms ) mass grams

1 mole (6.022 x atoms) of different elements g g g g g g

HW 5-1, #5 moles Number of atoms Element Mass in grams x Fe x Zn x S Al x Ar

HW 5-1 #6 Which of the following contains the same # of atoms as g of C? Which of the following contains the same # of atoms as g of C? Equal masses does NOT mean equal # of atoms because each type of atom has a different mass. Equal masses does NOT mean equal # of atoms because each type of atom has a different mass. Need to compare # of particles – chemists unit for # of particles = mole Need to compare # of particles – chemists unit for # of particles = mole How many moles are present in g? How many moles are present in g? Ans: 1 mole Ans: 1 mole C = 12 amu He = 4 amu

HW 5-1 #6 Question now becomes which of the following also represents 1 mole? Question now becomes which of the following also represents 1 mole? A) (1 mole = 6.94 g); g Li ≈ 2 moles A) (1 mole = 6.94 g); g Li ≈ 2 moles B) (1 mole = 4.00 g); 12.0 g He ≈ 3 moles B) (1 mole = 4.00 g); 12.0 g He ≈ 3 moles C) (1 mole = 10.8 g); 5.4 g of B ≈ 9 moles C) (1 mole = 10.8 g); 5.4 g of B ≈ 9 moles D) g Ne = mass on periodic table = 1 mole D) g Ne = mass on periodic table = 1 mole

HW 5-1 #6 Question now becomes which of the following also represents 1 mole? Question now becomes which of the following also represents 1 mole? A) (1 mole = 6.94 g); g Li ≈ 2 moles A) (1 mole = 6.94 g); g Li ≈ 2 moles B) (1 mole = 4.00 g); 12.0 g He ≈ 3 moles B) (1 mole = 4.00 g); 12.0 g He ≈ 3 moles C) (1 mole = 10.8 g); 5.4 g of B ≈ 9 moles C) (1 mole = 10.8 g); 5.4 g of B ≈ 9 moles D) g Ne = mass on periodic table = 1 mole D) g Ne = mass on periodic table = 1 mole

HW 5-1, #7 moles Number of atoms Element Mass in grams x He x He x He x He Ne Ne Ne Ne g 4.00 g 8.00 g g g g g g x x x x 10 24

What is the mass of 1 mole of Helium? What is mass of 1 mole of Bromine? x atoms of He x molecules of bromine (Br 2 ) 4.00 g He g Br 2

HW 5-2:What is the mass of 1 mole of water, H2O? Molar Mass of a compound =sum of masses of elements in formula H: 2 x 1.01 = 2.02 O: 1 x = H 2 O = g/mole

How to Use the Mole Bridge Two conversion factors: 1 mole 1 mole 1 mole x atomic mass (g) x atomic mass (g)

The Actual Bridge over River Mole or “under”