The Mole Concept

The Mole Concept

Review: Avogadro’s Number

• Avogadro’s Number (symbol N) is named in honor of Italian chemist Amadeo Avogadro

• It tells us the number of atoms in 1 mole of a substance

• The value of Avogadro’s number is 6.02x1023

Review: Mole

• The mole (symbol mol) is a unit of measure for an amount of a chemical substance•We define it as the amount of substance

that contains Avogadro’s number of particles• The individual particles can be atoms,

molecules, formula units, etc.

Review: Mole

• 1 Mole= Avogadro’s number (N)

• Avogadro’s number =6.02x1023 particles

• 1 Mole=6.02x1023 particles

Mole Analogies• If 6.02x1023 atoms of hydrogen were laid side by side, the total

length would be long enough to circle the Earth 1,000,000 times

• The mass of 6.02x1023 Olympic shot-put balls would equal the mass of the Earth

• The volume of 6.02x1023 softballs would equal the volume of the Earth

• Extra Credit Question: If $6.02x1023 earns 5% interest per year, how many dollars does the account earn every nanosecond? (must show all work to receive credit)

Mole Calculations: Nmoles

•We can convert between the number of particles and the number of moles.• Example:• How many moles of Iodine are in

2.5x1023 molecules of I2?

Mole Calculations: molesN

•We can convert between the number of moles and the number of particles.• Example:• How many molecules of chlorine are in

0.250 moles of the gas?

Molar Mass

•Molar mass is the mass of 1 mole of a substance• For elements the molar mass is equal to its

atomic mass, which we find on the periodic table• For compounds the molar mass is equal to

the sum of the individual elements that make it up

Molar mass examples

• Find the molar mass of the following substances:• Silver metal•Magnesium nitrate• NH3

•Manganese metal• Strontium acetate• Sulfur hexaflouride

Mole Calculations: gramsmol•We can convert between the mass of a

substance and the number of moles.• Example• How many moles are in 87.8g of Pb?

Mole Calculations: molgrams•We can convert between the mass of a

substance and the number of moles.• Example• How many grams are in 3.2 moles of

(NH4)3PO4 ?

Mole Calculations: g N

•We can also convert between mass and the number of particles• Example:• How many molecules are present in

1.470g of O2 gas?

Mole Calculations: g N

•We can also convert between the number of particles and mass• Example: • What is the mass in grams of 2.01x1022

atoms of sulfur?

Percent composition

• Percent composition is a list of the mass percent of each element in a compound• Before we continue we need to discuss

what percentages are.• Percent expresses the amount of a single

quantity compared to the entire sample

Calculating % composition

• Calculate the percent composition for each element in H2O

Calculating % composition

• Calculate the percent composition for each element in Trinitrotoluene (TNT), C7H5(NO2)3

Empirical Formula

• In the late 1700’s chemists were very interested in chemical reactions, especially those with oxygen.• This was because chemists could

determine the formula of a compound after the reaction through the use of empirical formulas• Empirical formulas are the simplest whole

number ratio of atoms in an element

Empirical Formula

Molecular Formula

Empirical Formula

H2O2

C6H12O6

CH3O

CH3OOCH = C2H4O2

Empirical Formula

• Glycine is an amino acid found in protein. An analysis of glycine gave the following data: 32.0% carbon, 6.7 % hydrogen, 18.7% nitrogen, and 42.6% oxygen. Calculate the empirical formula for the amino acid. (Assume 100g sample)

Empirical Formula

• Calculate the empirical formula for caffeine. An analysis of caffeine gave the following data: 49.5% carbon, 5.15 % hydrogen, 28.9% nitrogen, and 16.5% oxygen. (Assume 100g sample)

Molecular Formulas

• Molecular formulas indicate the actual numbers of atoms of each element in one molecule

• Benzene, styrene and acetylene, while very different substance with very different properties, all have the same empirical formula CH.

• Because they are different compounds they must have different formulas and therefore different molar masses.

Calculating molecular formulas

• Acetylene: 26 g/mole

Calculating molecular formulas

• Benzene: 78 g/mole

Calculating molecular formulas

• Styrene: 104 g/mole

Calculating molecular formulas

• The empirical formula for fructose is CH2O. If the molar mass of fructose is 180g/mol, find the molecular formula of fructose.

Calculating molecular formulas

• Ethylene dibromide was used as a grain pesticide until it was banned in 1983. • A. Find the empirical formula if the

percent composition is 12.7% C, 2.1% H and 85.1% Br.• B. If the molar mass is 180g/mol, find the

molecular formula of ethylene dibromide.