Mole Relationships

By definition: 1 atom 12C “weighs” 12 amu

Atomic mass is the mass of an atom in atomic mass units (amu)

The mole (mol) is the amount of a substance that contains as many elementary entities as there

are atoms in exactly 12.00 grams of 12C

Molar mass is the mass of 1 mole of in gramseggsshoes

marblesatoms

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

1 12C atom = 12.00 amu

1 mole 12C atoms = g 12C

1 mole lithium atoms = g of Li

For any element

atomic mass (amu) = molar mass (grams)

One Mole of:

C S

Cu Fe

Hg

Do You Understand Molar Mass?

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

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

SO2

1S 32.07 amu

2O + 2 x 16.00 amu SO2 64.07 amu

For any molecule

molecular mass (amu) = molar mass (grams)

Do You Understand Molecular Mass?

How many H atoms are in 72.5 g of C3H8O ?

Percent composition of an element in a compound =

n x molar mass of elementmolar mass of compound

x 100%

n is the number of moles of the element in 1 mole of the compound

C2H6O

Do You Understand Percent composition ?What is the percent composition of each element in C3H8O ?

Types of FormulasTypes of Formulas• Empirical FormulaEmpirical Formula

The formula of a compound that The formula of a compound that expresses the expresses the smallest whole number smallest whole number ratioratio of the atoms present. of the atoms present.

Ionic formula are always empirical formulaIonic formula are always empirical formula

• Molecular FormulaMolecular Formula

The formula that states the The formula that states the actualactual number of each kind of atom found in number of each kind of atom found in one one moleculemolecule of the compound. of the compound.

To obtain an To obtain an Empirical Empirical FormulaFormula

1.1. Determine the mass in grams of each Determine the mass in grams of each element present, if necessary.element present, if necessary.

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

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

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

** Be careful! Do not round off numbers prematurelyBe careful! Do not round off numbers prematurely

To obtain an To obtain an Empirical Empirical FormulaFormula

A sample of a brown gas, a major air pollutant, A sample of a brown gas, a major air pollutant, is found to contain 2.34 g N and 5.34g O. is found to contain 2.34 g N and 5.34g O. Determine an empirical formula for this Determine an empirical formula for this substance.substance.

To obtain a To obtain a Molecular Molecular FormulaFormula

Calculation of the Molecular FormulaCalculation of the Molecular Formula

A compound has an empirical formula of A compound has an empirical formula of NONO22. The colourless liquid, used in rocket . The colourless liquid, used in rocket

engines has a molar mass of 92.0 g/mole. engines has a molar mass of 92.0 g/mole. What is the What is the molecular formula molecular formula of this of this substance?substance?

Empirical Formula from % Empirical Formula from % CompositionComposition

A substance has the following composition by mass: 60.80 % Na ; 28.60 % B ; 10.60 % H

What is the empirical formula of the substance?

Molecular Formula from % Molecular Formula from % CompositionComposition

A substance has the following composition by mass: 60.80 % Na ; 28.60 % B ; 10.60 % H

What is the molecular formula of the substance if the molecular mass is 151.2 g/mol?

Combustion AnalysisMany times it is difficult to determine the mass of

individual elements. We can use combustion analysis of organic elements to determine the empirical formula for these compounds

1. Find total mass combusted

2. Find mass water using water absorption

3. Find mass carbon dioxide through CO2 absorption

4. Find remaining mass of other elements if needed.

Combust 11.5 g ethanol

Collect 22.0 g CO2 and 13.5 g H2O

Combust 11.5 g ethanol

Collect 22.0 g CO2 and 13.5 g H2O

If molar mass was found to be 46 g/mole what is the molecular formula?

Do You Understand Combustion analysis?

What is the empirical formula if 31.81 g CO2 and 11.39 g H2O were collected from a 15.0 g sample of an organic molecule containing carbon hydrogen and nitrogen?