Chemical Quantities
Rowland
Measuring Matter
• What do the questions “how much” and “how many”
have in common?
– They are questions about the amount of a substance and are
similar to questions scientist ask
• There are three common ways to measure matter
– Count the matter
– Measure the mass or weight
– Measure the volume
What is a Mole
• The SI unit for measuring the amount of a
substance is the mole
– A mole is a numerical amount of a substance
• 1 mole = 6.022 x1023 Representative Units which
is also known as Avogadro’s number
• Representative units or particles are the smallest
unit into which a substance can be broken down
without a change in composition
» Atoms, Molecules, Ions
How Large is Avogadro's Number?
• 6.02x1023
• 602,000,000,000,000,000,000,000
• If you could count two dollars every second, it
would take you 9,544,647,387,113,140 (Nine
Quadrillion, Five Hundred Forty Four Trillion, Six
Hundred Forty Seven Billion, Three Hundred
Eighty Seven Million, One Hundred Thirteen
Thousand, One Hundred Forty) years to count
one mole of dollars!
– The earth is only 4.5 billion years old!
Representative Units
Moles
• If you have 1.81x1024 atoms of gold, how many
moles do you have? (Atoms  Moles)
• If you have 1.81x1024 molecules of glucose
(C6H12O6), how many moles of glucose do you
have? (Molecules  Moles)
Moles
Representative Units
• How many atoms of gold do you have if you have 3.0
moles of gold? (Moles  Atoms)
• How many molecules of glucose (C6H12O6) do you have
if you have 3.0 moles of glucose? (Moles  Molecules)
• How many atoms are present in 3.0 moles of glucose
(C6H12O6)? (Moles  Molecules  Atoms)
The Mass of a Mole of Elements
• All elements have mass
– Larger elements are more
massive than smaller
elements
• The mass of one mole of
gold is 196.97 grams
• The mass of one mole of
hydrogen is 1.008 grams
• The mass of one mole
(6.02x1023) of an element
is its molar mass
– The molar mass of each
element is equal to the
atomic mass of an element in
grams
• The average atomic mass
of each element can be
found in the periodic table
Examples
Element
Atomic
Mass
(AMU)
Molar
Mass
(g/mol)
Hydrogen
(H)
1.0079
1.0079
Boron
(B)
10.81
10.81
Calcium
(Ca)
40.08
40.08
Bromine
(Br)
79.904
79.904
Uranium
(U)
238.03
238.03
Molar Mass Continued
How many atoms of mercury are in the beaker?
How many atoms of iron are on the plate?
Problems using molar mass (Single Element)
1. What is the molar mass of gold (Au)?
2. What is the mass of 3 moles of gold
(Au)? (Moles  Mass)
3. How many moles of gold are in 591
grams of gold (Au)? (Mass  Moles)
The Mass of a Mole of a Compound
1.
To calculate the molar mass of a compound, find the
number grams of each element in one mole of the
compound
2.
Then add the masses of the elements in the compound
Example – Determine the molar mass of acetic acid
(CH3COOH).
Molar mass of C 12.0 g/mol x 2 C atoms
= 24 g/mol C
Molar mass of H 1.00 g/mol x 4 H atoms
Molar mass of O 16.0 g/mol x 2 atoms
= 4 g/mol H
= 32 g/mol O
24 g/mol C + 4 g/mol H + 32 g/mol O = 60 g/mol CH3COOH
Problems using molar mass (compounds and molecules)
1. What is the molar mass of glucose (C6H12O6)?
2. What is the mass of 3 moles of glucose?
(Moles  Mass)
3. How many moles of glucose are in 540 grams
of glucose? (Mass  Moles)
Problems using molar mass (compounds and molecules)
1.
How many molecules of glucose are in 540 grams of
glucose (C6H12O6 / 180g/mole)? (Mass  Moles 
Molecules)
2.
How many atoms are in 540 grams of glucose? (Mass
 Moles  Molecules Atoms)
3.
How many hydrogen atoms are in 540 grams of
glucose (Mass  Moles  Molecules  Atoms of
Hydrogen)
Problems using molar mass (compounds and molecules)
• What is the mass of 1.81x1024 molecules
of glucose (C6H12O6)? (Molecules 
Moles  Mass)
Percent Composition and Chemical Formulas
• The relative amount of the elements in a
compound are expressed as the percent
composition or the percent by mass of each
element in the compound
• The percent by mass of an element in a
compound is the number of grams of the
element divided by the mass in grams of the
compound
% mass of element = mass of element x 100%
Mass of compound
Calculating percent composition from mass data
• When a 13-g sample of a compound containing only
magnesium and oxygen is decomposed, 5.40g of
oxygen is obtained. What is the percent composition of
the compound?
Calculating percent composition from chemical formula
% mass =
Mass of element in one mole of compound X 100%
Molar mass of a compound
1.
The subscripts in the formula of the compound are used to
calculate the mass of each element in a mole of that compound
2.
The sum of these masses is the molar mass
3.
Using the individual masses of the elements and the molar mass
you can calculate the percent by mass of each element in one
mole of the compound
4.
Divide the mass of each element by the molar mass and multiply
the results by 100%
Calculating percent composition from chemical formula
• Calculate the percent composition of ammonium nitrate
NH4NO3
• What is the percent nitrogen in ammonium nitrate
NH4NO3?
Empirical Formulas
• The empirical formula of a compound
gives the lowest whole-number ratio of the
atoms of the elements in a compound
– Take Glucose for instance
• Glucose has a molecular formula of C6H12O6
• Glucose has an empirical formula of CH2O
Determining empirical formulas of a compound
•
1,6-diaminohexane is used to make nylon. What is the empirical
formula of this compound if it is 62.1% C, 13.8% H, 24.1% N?
1.
Change the % to grams for each element
2.
Divide the mass of each element by its molar mass
3.
Divide the moles of each element by the element with the least
amount of moles
4.
The results of step 3 are the subscripts for each element in the
compound
Determining empirical formulas of a compound
• 1,6-diaminohexane is used to make nylon. What is the empirical
formula of this compound if it is 62.1% C, 13.8% H, 24.1% N?
Molecular formulas
•
The molecular formula of a compound is either
the same as its experimentally determined
empirical formula, or it is a simple wholenumber multiple of its empirical formula
– To determine the molecular formula of a compound
1. Divide the experimentally determined molar mass by the
molar mass of the empirical formula of the compound
2. Use the resulting multiplier to convert the empirical formula
into a molecular formula
Finding the molecular formula of a compound
• Find the molecular formula of ethylene glycol, which is
used as antifreeze. The molar mass is 62 g/mol and the
empirical formulas is CH3O.