The mole

What is a Mole
 Chemists use the mole to count microscopic particles.
 How many socks come in a pair?
 How many eggs are in a dozen?
 12
 How many eggs come in a gross?
 144
 How many pencils come in a ream?
 500
So how many atoms come in a
 602,213,670,000,000,000,000,000.
 Seriously
 This number was created by an Italian physicist and
lawyer named Amedeo Avogadro.
 Its called Avogadro’s Number.
 Is there an easier way to write it?
 6.02 x 1023 items = 1 mole
Converting between moles and
number of particles
 Think about eggs
 1 dozen eggs = 12 eggs
 Conversion factor:
 12 eggs/1 dozen eggs
 1 dozen eggs/12 eggs
 So if you have 3.5 dozen eggs how many eggs do you
 3.5 dozen x (12 eggs)/1 dozen = 42 eggs
 If you have 3.5 moles of sugar how many particles of
sugar do you have?
 1 mole of sugar = 6.02 x 1023 particles of sugar.
 Set up the problem:
 3.5 moles x (6.02 x 1023 particles/1 mole)
 Solve:
2.107 x 1024
 Zinc (Zn) is used to form a corrosion-inhibiting surface on
galvanized steel. Determine the number of Zn atoms in
2.50 moles of Zn.
1.51 x 1024 atoms
Calculate the number of molecules in 11.5 mol of water.
6.92 x 1024 molecules
Silver nitrate Ag(NO3)2 is used to make several different
compounds used in photographic films. How many
molecules of silver nitrate are there in 3.25 moles of
1.96 x 1024
How many atoms of oxygen are there in 5.0 mol of oxygen
6.02 x 1024 atoms
Converting from number of
particles to moles
 Calculate the number of moles of Zinc that contains
4.50 x 1024 atoms.
0.914 moles
How many moles can be made up from 5.57 x 1024
atoms of Al?
9.55 moles
How many moles can be made up from 2.5 x 1025 atoms
of Fe?
41.5 moles
 What would have more mass a dozen eggs or a dozen
Why would a dozen elephants have more mass?
Just like elephants and eggs certain atoms are bigger
than others.
For example Neon is much bigger than Helium.
Which would have more mass 1 mole of Neon or 1
mole of Helium?
Molar Mass
 The molar mass of an element is the mass in grams of
one mole of that element.
 Why is this important?
 Molar mass can be used to calculate the number of
atoms with out using a microscope.
Using molar mass
 If one dozen jelly beans has a mass of 35 g how much
mass does 5 dozen jelly beans have?
175 g
To convert from Moles to mass you multiply by the
molar mass.
What is the mass of 3.oo moles of copper?
191 g cu.
To convert from mass to moles you divide by the molar
Example Problems
 Chromium (Cr), a
transition element, is a
component of chrome
plating. Chrome plating
is used on metals and
steel alloys to control
corrosion. Calculate the
mass in grams of 0.045
moles of Cr.
 2.34 g Cr
 Calcium (Ca), the fifth most abundant element on
earth, is always found combined with other atoms
because of its high reactivity. How many moles of
calcium are in 525 g Ca?
 13.1 moles Ca
Converting between mass and
 The next step is converting a given mass of an element
into a number of atoms.
If we have 550 g of jelly beans and there are 35 g of jelly
beans in a dozen how many dozen jelly beans do we
16 dozen
How many jelly beans are there in 16 dozen?
Mass-to-Atom Conversion
 Gold (Au) is one of a group of metals called the
coinage metals. How many atoms of gold are there in a
U.S. gold eagle coin with a mass of 31.1 g?
 9.51 x 1022
Atom-to-Mass Conversion
 Helium (He) is an unreactive noble gas found
underground. A party balloon contains 5.5 x 1022 atoms
of helium gas. What is the mass, in grams, of the
 0.366 g
 Textbook (sorry)
 P. 328 # 15(a&b), 16(a&b)
 P. 329 # 17(a&b), 18(a&b)
 P. 331 # 19(a&b), 20(a&b)
Chemical Formulas and the Mole
 In the compound CCl2F2 how many atoms do we have?
 C: 1
 Cl: 2
 F: 2
 So if we have one mole of CCl2F2 we have one mole of
carbon, two moles of Chlorine and one mole of
Molar Mass of a Compound
 Determine the molar mass of each of the following
Converting moles to mass
 The characteristic odor of garlic is due to allyl sulfide
(C3H5)2S. What is the mass of 2.5 moles of allyl sulfide?
 What is the mass of 3.25 mol of H2SO4?
Converting Mass to Moles
 Calculate the number of moles in 325 g of Calcium
hydroxide Ca(OH)2?
 Calculate the number of moles in 22.6 g of AgNO3.
Mass to Number of Particles
 Aluminum chloride (AlCl3) is used in refining petroleum
and manufacturing rubber. How many Aluminum Chloride
molecules are present in 35.6 g of AlCl3
 How many Al atoms are there?
 How many Cl atoms are there?
 Aluminum oxide (Al2O3) when dissolved in water
breaks apart into ions (Al3+ and O2-) how many moles
of Al3+ will be produced when 1.5 moles of Al2O3 are
dissolved in water?
What is the molar mass of ethanol C2H5OH?
 How many ethanol molecules are present in 45.6 g?
 How many carbon atoms are there in 45.6 g of ethanol?
 How many hydrogen atoms are there in 45.6 g of ethanol?
How many oxygen atoms are there in 45.6 g of ethanol?
A sample of sodium sulfite Na2SO3 has a mass of 2.25 g
How many molecules of sodium sulfite are there?
How many atoms of sodium are there?
How many atoms of Sulfur are there?
How many atoms of oxygen are there?
Percent Composition
 A compound is made up of one or more atoms bonded
 Each of the atoms contributes mass to the compound.
 Example:
 The molar mass of NaOH is
 Na – 22.98 g/mol
 O – 16 g/mol
 H – 1 g/mol
 Total: 39.98 g/mol
Calculating Percent Composition
 Percent composition is calculated by dividing the mass
of an individual element by the mass of the whole
compound and then multiplying by 100.
 NaOH:
 Percent Composition of Na:
 (22.98/39.98) x 100 = 57.5%
 Percent Composition of O:
 (16/39.98) x 100 = 40%
 Percent Composition of H:
 (1/39.98) x 100 = 2.5%
Compounds with multiple atoms of
the same element
 NaHCO3
 Molar Mass: 84.01 g/mol
 Percent Composition of Na:
 (22.98/84.01) x 100 = 27.37%
 Percent Composition of H:
 (1/84.o1) x 100 = 1.2%
 Percent Composition of C:
 (12.01/84.01) x 100 = 14.3%
 Percent Composition of O:
 (48/84.01) x 100 = 57.14%
Empirical Formula
 When we know a compounds percent composition we can determine
it’s formula.
The empirical formula of a compound is the formula with the smallest
whole-number ratio of elements.
Find the empirical formula for a compound that is 40.05% S and
59.95% O.
If we assume that we have 100 g of this compound:
How many grams of Sulfur do we have?
 40.05 g
 How many grams of Oxygen do we have?
 59.95 g
 Convert from grams to moles:
 40.05 g S x 1 mol S/32.07 g S = 1.249 mol
 59.95 g O x 1 mol O/16.00 g O = 3.747 mol
 S1.249O3.747
 SO3
 Sometimes the empirical formula is not the actual
formula for the compound.
 Find the empirical formula for hydrogen peroxide
which has a molar mass of 34 g/mol and is 5.9%
Hydrogen and 94.1% Oxygen
Molecular Formula
 A compounds molecular formula is its actual formula
(not always the same as empirical formula)
 To determine the true molecular formula for a
compound its molar mass must be known (either
looked up or found experimentally).
 Once we know the molar mass and the empirical
formula we can find the true formula of the
 Example:
 Acetylene is a highly flammable compound used in blow
torches. Acetylene has a molar mass of 26.04 g/mol. It is
92.2% Carbon and 7.8% Hydrogen. What is the chemical
formula of Acetylene?
 Step 1: Find empirical formula:
 CH
 Step 2: Calculate molar mass of empirical formula. (If the
molar mass is the same as the actual molar mass then the
empirical formula is the true formula.)
 13.02 g/mol
 Step 3: Divide real molar mass by the molar mass of the
empirical formula.
 26.04/13.02 = 2.000
 Step 4: Multiply all atoms in the empirical formula by the
answer to the true formula.
 C2H2

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