Chapter 8

The Mole

2

I say, “I have a dozen….”• You say....

✓A dozen what?

✓eggs, donuts, pencils, dogs, siblings....whatever.

• What is a dozen?

✓12 items

• How would you label a dozen?

✓12 items/1 dozen

3

I’ll give you a dozen rice…

• You might say, “Big deal.”• This would not be useful because rice grains are so

small.• You would still be hungry.

4

OK, I’ll give you a dozen gold atoms…

• You might again say, “So what.”• This would not be valuable because a dozen atoms

is so small, so in chemistry we need a much LARGER dozen.

• Introducing...

The really big dozen

The Mole

6

Using the Mole Just Like the Dozen• If you had 1 dozen bicycles, you’d have… • how many bicyles, dozen frames, dozens of

wheels, frames, wheels?✓1 dozen bikes*12 bikes/1doz = 12 bikes

✓1 dozen bikes*1frame/1bike = 1 dozen frames

✓1 dozen bikes*2 wh/1 bike = 2 dozen wheels

✓1 dozen bikes*12 bikes/1doz*1frame/1bike✴ 12 frame

✓1 dozen bikes*12 bikes/1doz*2wheels/1bike✴ 24 wheels

7

The Chemist’s Dozen - A MoleAvogadro’s Number of “items”

• 6.022 141 99 x 1023 items/mol✓Atoms or molecules or ions or units or whatever

(dogs, donuts, eggs, bicycles, etc)

• 602,214,199,000,000,000,000,000 items/mol• The average atomic mass of any element is

the mass of a mole of atoms of that element.✓H = 1.01 g/mol✓C = 12.01 g/mol✓Fe = 55.85 g/mol

thou

sand

s

milli

ons

billio

ns

trillio

ns

quad

rillio

ns

quin

trillio

ns

sext

illion

s

8

Molar Mass - The Average Mass in the Periodic Table

• The mass (in grams) of 1 mole of a substance.

• Determine the MM (molar mass) of carbon tetrachloride: CCl4✓1 mole C = 12.01 g✓4 mole Cl = 4 (35.45 g) = 141.8 g✓12.01 g + 141.8 g = 153.81 g/1 mol of CCl4 molecules

9

Molar Mass, again• Determine the MM (molar mass) of aluminum

hydroxide: Al(OH)3

✓1 mole Al = 26.98 g✓1 mole O = 16.00 g * 3 = 48.00 g✓1 mole H = 1.01 g * 3 = 3.03 g✓26.98 g/1mol + 48.00 g/1mol + 3.03 g/1mol

78.01 g/1mole of Al(OH)3 formula units aka ionicules

10

Molar Mass of Large Compounds• Determine the MM (molar mass) of iron(III) sulfate: Fe2(SO4)3 ✓2 mole Fe = 2 (55.85 g) = 111.70 g✓3 mole S = 3 (32.07 g) = 96.21 g✓12 mole of O = 12 (16.00) = 192.00g✓111.70 g + 96.21 g + 192.00 g =

399.91 g/1mol of iron(III) sulfate ionicules

• So if you had 5 moles of iron(III) sulfate, how much would it weigh?✓5 moles * ~400 g/1mol = ~2000 g

~2000 g of iron(III) sulfate

11

So what is the unit label on the molar masses that you look up in the

periodic table?

• g/mol • (or g/1 mole)

12

So what is the unit label on Avogadro’s number?

6.02 x 1023

• items/mol

13

Mass ↔ Moles• How many moles of Mg in 75.0 g of Mg?✓1 mole Mg = 24.31 g✓(75 g)(1 mol/24.31g) = 3.085150144 moles

3.09 moles of Mg atoms

• What is the mass of 0.732 mole of Cl2 molecules?✓1 mole Cl atoms = 35.45 g✓1 mole Cl2 molecules = 70.90 g✓(0.732 mole)(70.90 g/1mole) = 51.8988 g

51.9 g of Cl2 molecules

14

Moles ↔ Items• How many atoms are in 4.7 moles of Mg?✓1 mol Mg = 6.02 x 1023 atoms✓(4.7 mole)(6.02 x 1023 atoms/1mol) =

2.8294 x 1024 atoms2.8 x 1024 atoms of Mg

• How many mole of oxygen molecules in 5.83 x 1022

molecules?✓1 mole O2 = 6.02 x 1023 molecules✓(5.83 x 1022 molecules)(1 mol/6.02 x 1023 molecules)

0.096843853 mole0.0968 mole of O2 molecules

15

Mass ↔ Moles ↔ Items• What is the mass of 2.8 x 1024 atoms of Mg?✓1 mole Mg = 6.02 x 1023 atoms, 1 mole Mg = 24.31 g✓This is a two step process: ✓Change to moles, then change to grams✓(2.8 x 1024 atoms)(1 mole/6.02 x 1023 atoms)(24.31 g/1mole) =

113.0697674 g110 g of Mg atoms

• How many ammonia molecules are in 34 g of NH3?✓1 mole NH3 = 17.04 g, 1 mole = 6.02 x 1023 molecules✓This is a two step process:✓Change to moles, then change to molecules✓(34 g)(1 mole/17.04 g)(6.02 x 1023 molecules/1mole) =

1.201173709 x 1024 molecules1.2 x 1024 molecules of NH3

Practice Problem• What would be the mass of 3.75 x 1021 atoms

of iron?✓Change to moles, then change to grams✓(3.75 x 1021 atoms)(1 mole/6.02 x 1023 atoms)(55.85 g/1mole) =

0.3479028 g0.348 g of Fe atoms

16

Practice Problem

17

• How many water molecules would be found in a 54 gram sample of water?✓1 mole H2O= 18.02 g, 1 mole = 6.02 x 1023 molecules✓This is a two step process:✓Change to moles, then change to molecules✓(54 g)(1 mole/18.02 g)(6.02 x 1023 molecules/1mole) =

1.80459 x 1024 molecules1.8 x 1024 molecules of H2O

18

Sometimes in chemistry it is useful to report the percentages of elements in a compound. This is called:

Percent Composition (by mass)aka Elemental Analysis

• Determine the % composition by mass✓(aka an elemental analysis of MgF2)

• Mg = 24.31 g/1mol, F = 19.00 g/1mol• MgF2 = 62.31 g/1mol• Remember that % is✓always part out of total

• Mg = (24.31 / 62.31)(100) = 39.01%• F (all of it) = (38 / 62.31)(100) = 60.99%

Determine the Percent Composition of carbon in the following:

19

1. CO2

C = 12.01 g/1mol, O = 16.00 g/1mol MM = 12.01+16.00(2)= 44.01 g/molSo % C = (12.01 / 44.01)(100) = 27.29%

2. C6H12O6

MM = 12.01(6) +1.01(12) +16.00(6) =180.g/molSo % C = (72.06/180)(100) = 40.0%

20

Elemental Analysis