Unit 6

MolesConversions

Formulas

Mole

• SI base unit for measuring the amount of substance

• The number of representative particles in exactly 12 grams of pure carbon-12

• 1 mole = 6.02 x 1023 representative particles

Avogadro’s Number

• 6.02 x 1023 is called Avogadro’s number• It is the number of representative particles

equal to 1 mole• A representative particle is any kind of particle

and depends on the substance– Water = molecule– Copper = atom– NaCl (salt) = formula unit

Molar/Formula Mass

• Molar/Formula mass is the mass in grams of 1 mole of any pure substance. – units are g/mol

• Molar mass is equal to the atomic mass on the periodic table

• Gram atomic mass: (found on periodic table)– Ex: C = 12.011 amu

• This means 1 mole of Carbon = 12.011 g

• Let’s practice: Find the # of grams 1 mole of the following is equal to– Al– H– O– P

Formula Mass/Molar Mass• Formula or Molar mass is the mass of 1 mole

of a pure substance

H2O – covalent compound (nonmetal/nonmental)

H = 2 x 1 g = 2 gO = 1 x 16 g = 16 gtotal = moles x avg. atomic mass= 18 g

• Let’s practice: Find the # of grams 1 mole of the following is equal to– H2O2

– Sodium chloride– Calcium nitrate– (NH4)3PO4

– Dinitrogen tetraoxide

Percent Composition

• You can calculate the percentage of a certain element in a compound

• Use the % Formula: (Part/Whole) x 100 =

• 1st- we need the formula mass or the whole• 2nd – we need the part found in the compound• 3rd – divide and multiple by 100

• Example: What is the % of Hydrogen in H2O?

1st: Gram atomic Mass– H2O – covalent compound (nonmetal/nonmental)

H = 2 x 1 = 2O = 1 x 16 = 16total = moles x avg. atomic mass= 18 g

2nd: Hydrogen has a mass of 2g total

3rd: %H = (2g/18g) x 100 = 11.11%

• What is the percent of oxygen in in H2O?

• The percent of each element in a compound should add and equal 100.

• Lets practice: Find the percent composition of each of the elements in the following compounds. – Fe2O3

– Copper (I) sulfide

Molar Mass can be used as a Conversion Factor

• What is the mass in grams of 2.5 mol of NaCl?

• Steps– 1st : get the formula/molar mass of the compound– 2nd: convert

Na = 1 x 23 = 23Cl = 1 x 35.5 = 35.5

= 58.5g

?g = 2.5 mol NaCl 58.5 g NaCl = 146 g NaCl 1 mol NaCl

Avogadro’s number as a conversion factor

• 1 mole = 6.02 x 1023 representative particles• Representative particles can be atoms, molecules,

particles, formula units, pieces, etc• How many molecules are in 3.5 moles of CO2?

?molecules = 3.5 mol CO2 6.02 x 1023 molecules =1 mol NaCl

2.1 x 1024 molecules CO2

Volume Conversions

• At STP 1 mol = 22.4 liters of a gas• What volume will 54.6 grams of CO2 occupy at

STP?

?liters = 54.6 g CO2 1 mol CO2 22.4 liters CO2 =44 g CO2 1 mole CO2

27.8 liters of CO2

Bell Work• Complete the following on a NEW sheet of paper

to TURN IN!

1.How many grams are in 0.817 moles of C2H2O6?

2.How many moles are in 60 grams of CaC03?

3.How many formula units of CaCl2 are in 45 grams?

4.How many molecules of CO2 are in 4.3 moles?

Empirical & Molecular Formulas• Empirical Formula - the simplest whole number

formula/ratio of a compound. This means the equation cannot be reduced anymore. (Smallest whole number ratio)

• Molecular Formula – the actual or true formula of the compound – indicates the actual number of atoms in the compound

• Sometimes the empirical and molecular formula are the same. If they are different the molecular is ALWAYS larger.

Calculating Empirical Formulas

• Example: An unknown sample contains 25% H and 75% C. What is the empirical formula?

• Steps: 1st Convert % to grams

(Assume you have 100 grams total)

2nd Convert grams to moles by using the atomic mass

25 % = 25 grams H75 % = 75 grams C

25 % = 25 g H x (1mol H/1g H)= 25 mol H

75 % = 75 g C x (1mol C/12 g C)= 6.25 mol O

3rd Divide by the smallest # of moles to get the ratio

4th Put numbers as subscripts of elements in formula

5th if a whole number is not found after dividing, multiply by a factor to make it a whole # (must multiply all numbers by the same factor)

H = 25 mol/ 6.25 mol = 4

C = 6.25 mol / 6.25 mol= 1

CH4

Molecular Formula

• Indicates the actual amount of atoms present

• Must have molecular weight to determine molecular formula

Determining Molecular Formula

Steps: 1.Determine the empirical formula2.Determine the empirical weight (molar mass

of the empirical formula)3.Divide the molecular weight given by the

empirical weight you calculated– If one, then the empirical formula is the same as

the molecular formula

4. If the answer from step 3 is a whole number, then take the answer and multiply all subscripts in the equation by the number.

5. If it is not a whole number you must multiple by a factor to make it a whole number then multiply each of the subscripts by it.

This will give the molecular formula.

Molecular Formula Example

• What is the molecular formula of a compound with a molecular mass of 64 g/mol and an empirical formula of CH4?

• CH4 empirical mass = (12) + (4 x 1) = 16g/mol

• Molecular mass = 64 g/mol• 64 / 16 = 4• Multiply subscripts by 4• New formula is C4H16

Hydrates

• A compound that contains a certain amount of water molecules

• Example: CuSO4 . 5H2O

• Copper sulfate pentahydrate

• Hydrates can be dehydrated by heating• The dehydrated form can be used to absorb

moisture from the air – like those packets you find in your shoe boxes – deliquescent