using chemical formulas

Bring your calculators to class

Remember the mole? (not just a furry animal that digs

holes in the yard.)

•unit used by chemist to measure things.•1 mole is 6.02 x 1023 particles (like a very

large dozen used for very small things)

•Defined as the number of carbon atoms in exactly 12 grams of carbon-12.

•Makes life very convenient

What sort of things are measured in moles?

Representative particles(The smallest pieces of a substance.)

For a molecular compound it is a molecule.

H2O

For an ionic compound it is a formula unit.

NaCl

For an element it is an atom.Au

+

- +

+

+---

Molar Mass generic term for the mass of one mole. Same as: Don’t WriteDon’t Write

1.gram atomic mass =The mass of 1 mole element in grams

2.gram formula mass =the mass of 1 mole of one formula unit is the sum of the atomic masses of all atoms of element

3.gram molecular mass =the mass of 1 mole of one molecule is the sum of all the atomic masses of all the atoms of elements in the molecule

1. Elements

The mass of 1 mole element in grams

1 mole of any element = atomic mass on periodic table

example: 1 mole of Na=23 g (22.989768 rounded to whole #)

1 mole of Cl = 35g

(35.4527 rounded to 35)

2. Ionic compounds

the mass of 1 mole of one formula unit is the sum of the atomic masses of all atoms of element

example: 1 mole of NaCl= 23 mass of Na

+ 35 mass of Cl

58 g

3. Molecular compounds

the mass of 1 mole of molecules is the sum of all the atomic masses of all the atoms of elements in a molecule

example: 1 mole of H2O2=

2 x 1g = 2g mass of H

2x16 + 32g mass of O

34g

Examples

Calculate the molar mass of the following and tell what type it is.

Na2S

N2O4

C

Ca(NO3)2

C6H12O6

Formula Mass = Molar Mass

Sum of the atomic masses of all the atoms in a chemical formula

H2O 2 H atoms each 1g= 2x1= 2

1 O atom 16 g +16

18 g 1mole of H2O = 18 g1 mole = 6.022 x 1023 particles

Using Molar Mass

Molar Mass =The number of grams of 1 mole of atoms, formula units, or molecules.

We can make conversion factors from these.

To change grams of a compound to moles or moles to grams of a compound use 1 mole= molar mass to make factor

Molar mass for NaOH

1mole Na = 23g

+1 mole O = 16g

+1 mole H = 1g

1 mole NaOH = 40g

5.69g NaOH 1mole NaOH

40 g NaOH

= 0.14 mole

For example: mass to moles

How many moles is 5.69 g of NaOH?

need to change grams to moles

Another Type: moles to massHow many grams are 2 moles H2O? Need to change from moles to grams

Molar mass of H2O

2 x 1g = 2 g of H

+16g of O

18g

2 moles H2O 18 g H2O

1 mole H2O

=36 g H2O

1 mole of H2O= 18g

Other Types of questions

How many molecules of CO2 are the in 4.56 moles of CO2 ?

1 mole = 6.02 x 1023 particles

4.5 moles 6.02 x 1023 molecules = 1 mole

2.79 x 1024 molecules

How many moles is 7.78 x 1024 formula units

of MgCl2?

7.78 x 1024 formula units 1 mole

6.02 x 1023 formula units

1.29 x 10 or 12.9 moles

Examples

How much would 2.34 moles of carbon weigh?

How many moles of magnesium in 24.31 g of Mg?

How many atoms of lithium in 1.00 g of Li?

How much would 3.45 x 1022 atoms of U weigh?

Hydrates- when some salts crystallize from a water solution and they bind water molecules in their crystal structure

Cu SO4•5H2O there are 5 water molecules for every copper(II) sulfate formula unit

copper(II) sulfate pentahydrate

Heating the crystal in a crucible drives off the water then the salt is called anhydrous

CuSO4•5H2O CuSO4 + 5H2O

CuCl2•2H2O CuCl2 + 2H2O

Example: Calculate the percent composition of Na in NaCl?

% Composition Like all percents part x 100 % = %

whole

1. Find the mass of each component

2. divide by the total mass x 100 %

mass Na x 100 % = 23 g x 100 % =39%mass NaCl 23 + 35

Empirical Formulas

-simplest whole number ratio of atoms

% to mass

mass to mole

divide by small

multiply ‘til whole

1. % to mass

assume 100 g so % = mass

53.73 g Fe & 46.27 g S

Example

What is the empirical formula of the compound that contains 53.73 % Fe and 46.27 % S?

2. Mass to mole

1 mole Fe = 55.85 g

53.73 g Fe 1mole = .96 mole Fe

55.85 g Fe

1 mole S = 32.06 g

46.2 g S 1 mole S = 1.44 mole S

32.06 g S

3. Divide by small

Fe .96/.96 = 1

S 1.44/.96 = 1.5

4. Multiply ‘til whole

Fe 1 x 2 = 2

S 1.5 x 2 = 3

Write empirical formula Fe2S3

Molecular formula = How the molecule actually existsIf the molar mass of the compound is known, then the molecular formula can be determined from the empirical formula. 1. Add the masses of all the atoms of each elements in the empirical formula.2.Divide the molar mass by the mass determined in step 1.3.Multiply each subscript in the empirical formula by the number calculated in step 2.