Mass Relationships in Mass Relationships in Chemical ReactionsChemical Reactions

Chapter 3 Chapter 3

Formula Weight (FW)• Sum of the atomic weights for the atoms in a

chemical formula.

• e.g., formula weight of calcium chloride, CaCl2

Ca: 1(40.1 amu)

+ 2Cl: 2(35.5 amu)

111.1 amu

• Formula weights used for ionic or molecular compounds

Molecular Weight (MW)

• Sum of the atomic weights of the atoms in a molecule.

• For the molecule ethane, C2H6, the molecular weight would be

C: 2(12.0 amu)

30.0 amu

+ 6H: 6(1.0 amu)

• Formula weights used for molecular compounds only

Percent Composition

Percentage mass of a compound that comes from each of the elements in the compound:

% element =(number of atoms)(atomic weight)

(FW of the compound)x 100%

Percent Composition

For the molecule ethane, C2H6, with MW = 30.0 amu

%C =(2)(12.0 amu)

(30.0 amu)

24.0 amu

30.0 amu= x 100%

= 80.0% C in ethane

The mole (mol) - amount of a substance that contains as many particles as there

are atoms in exactly 12.00 grams of 12C

1 mol = 6.02 x 1023

Avogadro’s number (NA)

Micro World(atoms & molecules)

Macro World(grams)

Avogadro’s Number• 1 carbon atom has a mass of 12.0 amu• 1 mole of 12C has a mass of 12.0 g

Micro World(atoms & molecules)

Macro World(grams)

Using Moles

• Moles provide a bridge from the molecular scale to the real-world scale.

How many moles of calcium (Ca) are in5.0750 g of calcium? (Z = 20)

From periodic table: 1 mol Ca = 40.078 g Ca

Ca mol 0.12263Ca g 40.078

Ca mol 1Ca g 5.0750

Molar mass - the mass of 1 mole of in grams

atoms

moleculesions

1 mole 12C atoms = 6.022 x 1023 atoms = 12.00 g

1 mole S atoms = 32.066 g S

1 mole lithium atoms = 6.941 g of Li

For any element:

atomic mass (amu) = molar mass (grams)

formula units

(Average) atomic mass (6.941)

Do You Understand Molar Mass?

How many moles are in 0.551 g of potassium (K) ?

1 mol K = 39.0983 g K

0.551 g K 1 mol K39.0983 g K

x = 1.41 x 10-2 mol K

Molecular mass (or molecular weight) - the sum ofthe atomic masses (in amu) in a molecule.

1S 32.07 amu

2O + 2 (16.00 amu) SO2 64.07 amu

For any molecule:

molecular mass (amu) = molar mass (grams)

1 molecule SO2 = 64.07 amu

1 mole SO2 = 64.07 g SO2

SO2

Do You Understand Molar Mass?

How many moles are in 72.5 g of C3H8O ?

1 mol C3H8O = (3 mol C) (12.0 g/mol) + (8 mol H) (1.0 g/mol) + (1 mol O) (16.0 g/mol)

1 mol C3H8O = 60.0 g C3H8O

(72.5 g C3H8O) 1 mol C3H8O

60.0 g C3H8O = 1.21 mol C3H8O

Do You Understand Molar Mass?

What is the mass of 0.433 mol of Ca(NO3)2 ?

1 mol Ca(NO3)2 = (1 mol Ca) (40.078 g/mol)

+ (2 mol N) (14.01 g/mol)

+ (6 mol O) (16.00 g/mol)

= 164.1 g Ca(NO3)2

0.433 mol Ca(NO3)2 · 164.1 g Ca(NO3)2

1 mol Ca(NO3)2

= 71.1 g Ca(NO3)2