Unit 8 – The Mole

Essential Questions:•What is the relationship between a mole of a substance and its mass?•How can the mole of a substance be calculated?•How can the percent composition of a compound be determined?•How does the molecular formula of a compound compare with the empirical formula?

Formula Mass

•The sum of the average atomic mass for all atoms in represented in a formula•Unit is atomic mass units (amu)

1 atom of C = 12.01 amu

1 atom of Mg = 24.31 amu

1 atom of Cu = 63.55 amu

Molecular Mass – the sum of the masses of all the atoms in a molecule of a substanceThe unit is amu.

CaCO3

1 atom of Ca = 40.08 amu1 atom of C = 12.00 amu3 atoms of O = 3 x 16.00 amu

100.08 amu

Example:

Find the molecular mass of NH4SO2

1 N = 14.01 amu4 H = 4(1.01 amu)1 S = 32.07 amu2 O = 2(16.00 amu)

1 molecule = 120.7 amu

Try these problems:

1.HNO3

2.C6H10O5

3.H2SO4

= 63.01 amu

= 162.16 amu

= 98.08 amu

Mole

•A counting unit

•6.02 X 1023 (in scientific notation) •This number is named in honor of Amedeo Avogadro (1776 – 1856)Amedeo Avogadro (1776 – 1856), who studied quantities of gases and discovered that no matter what the gas was, there were the same number of molecules present in the same volume

Mole – 6.02 x 1023

particles

1 mole C

1 mole H2O

1 mole NaCl

= 6.02 x 1023 C atoms

= 6.02 x 1023 H2O

molecules

= 6.02 x 1023 NaCl formula units

6.02 x 1023 Na+ ions and

6.02 x 1023 Cl– ions

Avogadro’s Number as Conversion Avogadro’s Number as Conversion FactorFactor

Particles = Moles 6.02 x 1023 particles

1 mole

Or Moles = Particles 1 mole

6.02 x 1023 particles

Note that a particle could be an atom

OR a molecule!

You MUST use dimensional analysis for

conversions!

X

X

Examples:

How many molecules are in 3.5 moles of H2O?

How many moles are present in 465 molecules of NO2?

How many atoms of nitrogen are in 3.15 moles of NH3?

How many atoms of chlorine are in .862 moles of MgCl2?

Molar Mass

Molar Mass- the mass of one mole of a substance

Unit is grams/mole Equivalent to the molecular mass in

amu

Ex: molar mass of Iron = 55.85 g /mole molecular mass of Iron = 55.85 amu

Mass and Mole Relationships

Examples:

1.Find the number of moles present in 56.7 g of HNO3.

2.Find the number of grams present in 4.5 moles of C6H10O5.

3.Find the number of moles present in 12.31 g of H2SO4.

Percent Composition

•Finding what percent of the total weight of a compound is made up of a particular element

Formula for calculating % composition:

Total amu of the element in the compoundTotal formula amuX 100%

Example:

Calculate the % composition of BeO

Example:

Calculate the % composition Ca(OH)2

Example:

Calculate the % composition of Al(NO3)2

Chemical Formulas

Formulas give the relative numbers of Formulas give the relative numbers of atoms or moles of each element in a atoms or moles of each element in a formula unit - always a whole number formula unit - always a whole number ratio (ratio (the law of definite proportionsthe law of definite proportions).).

1 molecule NO1 molecule NO22 : 2 atoms of O for every : 2 atoms of O for every 1 atom of N1 atom of N

1 mole of NO1 mole of NO22 : 2 moles of O atoms to : 2 moles of O atoms to every 1 mole of N atomsevery 1 mole of N atoms

Law of Multiple Proportions

When any two elements, A and B, combine to form more than one compound, the different masses of B that unite with a fixed mass of A bear a small whole-number ratio to each other

Example:In H2O, the proportion of H:O = 2:16 or 1:8In H2O2, H:O is 2:32 or 1:16

Empirical Formula - The formula of a compound that expresses the smallest whole number ratio of the atoms present.

Ionic formulas are always empirical formulas

Molecular Formula - The formula that states the actual number of each kind of atom found in one molecule of the compound.

Determine the Empirical Formula From the Molecular Formula Reduce!!

C6H6

Fe3(CO)9

BaCl2 P4O10

Determine the Molecular Formula from the Empirical Formula Calculate the molar mass of the

Empirical Formula. Divide the molar mass of the

Molecular Formula by the molar mass of the Empirical Formula

Multiply the numbers of each type of atom by that number

Determine the Molecular Formula from the Empirical Formula Examples:

Molecular Formula: 26.04 g/mol Empirical Formula: CH

Molecular Formula: 380.88 g/mol Empirical Formula: SeO3

To Obtain Empirical FormulaTo Obtain Empirical Formula

1.1. Assume the percent is out of Assume the percent is out of 100 grams. That means you 100 grams. That means you can change the % sign to can change the % sign to grams.grams.

2.2. Calculate the number of Calculate the number of molesmoles of of each element.each element.

3.3. Divide each by the smallest Divide each by the smallest number of moles to obtain number of moles to obtain the the simplest whole number simplest whole number ratio.ratio.

4.4. If whole numbers are not If whole numbers are not obtainedobtained** in step 3), in step 3), multiply through by the multiply through by the smallest number that will smallest number that will give all whole numbersgive all whole numbers

**Remember this**Percent to massMass to moleDivide by smallMultiply 'til whole

Calculating Empirical Formula Example:

1. Given that a compound is composed of 60.0% Mg and 40.0% O, find the empirical formula.

Calculating the Empirical Formula Example #2:

A compound is analyzed and is found to contain 13.5g of calcium, 10.8g of oxygen, and 0.675g of hydrogen. Calculate the empirical formula of this compound.

Calculating the Empirical Formula Example #3:

NutraSweet is a zero calorie sweetener used in many food products. A sample is analyzed and it’s percent composition is as follows; 57.14% carbon, 6.16% hydrogen, 9.52% nitrogen, and the rest is oxygen. Calculate the empirical formula of NutraSweet.

Try this!

A compound is found to contain 68.5% carbon, 8.63% hydrogen, and 22.8% oxygen. The molecular weight of this compound is known to be approximately 140.00 g/mol. Find the empirical and molecular formulas.