Counting Atoms
Chemistry is a quantitative science—we need a
“counting unit.”
MOLE - 1 mole is the amount of
substance that contains as many
particles (atoms, molecules) as there
are in 12.0 g of 12C.
Avogadro’s Number
6.02214199 x 1023
There is Avogadro’s number of
518 g substance.
of Pb, 2.50 mol
particles in a mole of any
Amedeo Avogadro
1776-1856
Molar Mass
1 mol of 12C
= 12.00 g of C
= 6.022 x 1023 atoms of C
12.00 g of 12C is its MOLAR MASS
Taking into account all of the
isotopes of C, the molar mass of
C is 12.011 g/mol
One-mole
Amounts
PROBLEM: What amount of Mg is
represented by 0.200 g? How many atoms?
Mg has a molar mass of 24.3050 g/mol.
PROBLEM: How many hydrogen atoms in 0.036 moles
of propane C3H8? In 2.7 moles?
Atomic Weights
• Weighted average of the masses of
the constituent isotopes if an
element.
– Tells us the atomic masses of
every known element.
– Lower number on periodic chart.
• How do we know what the values of
these numbers are?
MOLECULAR WEIGHT & MOLAR MASS
Molecular weight = sum of the atomic weights of all
atoms in the molecule.
Molar mass = molecular weight in grams
Problem: What is the molar
mass of ethanol – C2H6O?
1 mol contains
2 mol C (12.01 g C/1 mol) =
6 mol H (1.01 g H/1 mol) =
1 mol O (16.00 g O/1 mol) =
TOTAL = molar mass =
Tylenol
• Formula = C8H9NO2
• Molar mass =
PROBLEM: How many moles of alcohol
are there in a “standard” can of beer if
there are 21.3 g of C2H6O?
(a) Molar mass of C2H6O = 46.08 g/mol
(b) Calc. moles of alcohol
PROBLEM: How many molecules of
alcohol are there in a “standard” can of
beer if there are 21.3 g of C2H6O?
We know there are 0.462 mol of C2H6O.
PROBLEM: How many atoms
of C are
there in a “standard” can of beer if there
are 21.3 g of C2H6O?
Empirical & Molecular Formulas
A pure compound always consists of the same elements combined
in the same proportions by weight.
Therefore, we can express molecular composition as PERCENT
BY WEIGHT
Ethanol, C2H6O
52.13% C
13.15% H
34.72% O
Percent Composition
Consider NO2, Molar mass = ?
What is the weight percent of N and of O?
What are the weight percentages of N and O in NO?
A compound of B and H is 81.10% B. What is its empirical formula?
1.
Calculate the number of moles of each element in 100.0 g of sample.
2.
Take the ratio of moles of B and H. Always divide by the smaller number.
3.
Now, recognize that atoms combine in the ratio of small whole numbers.
4.
Find the ratio of moles of elements in the compound.
We need to do an EXPERIMENT to find the MOLAR MASS. Experiment gives 53.3 g/mol
Compare with the mass of B2H5 = 26.66 g/unit , i.e. Find the ratio of these masses.
Data to Determine the formula of a Sn—I Compound
Sn(s) + some I2(s) ---> SnIx
•
•
•
•
Reaction of Sn and I2 is done using excess Sn.
Mass of Sn in the beginning = 1.056 g
Mass of iodine (I2) used = 1.947 g
Mass of Sn remaining = 0.601 g
•
•
•
•
•
•
•
•
Find the mass of Sn that combined with 1.947 g I2.
Mass of Sn initially =
Mass of Sn recovered =
Mass of Sn used =
Find moles of Sn used:
Now find the number of moles of I2 that combined with Sn.
How many mol of iodine atoms?
Now find the ratio of number of moles of moles of I and Sn that combined.
Chemical Equations
• Look at the information an equation provides:

Fe2 O3 + 3 CO  2 Fe + 3 CO 2
Can work in moles, formula units, etc.
Frequently, we work in mass or weight (grams or kg or pounds or tons).
Chemical Equations
• Law of Conservation of Matter
– There is no detectable change in quantity of matter in
an ordinary chemical reaction.
– Balanced chemical equations must always include the
same number of each kind of atom on both sides of the
equation.
– This law was determined by Antoine Lavoisier.
• Propane,C3H8, burns in oxygen to give carbon
dioxide and water.

C3H 8  5 O 2 
 3 CO 2  4 H 2 O
Law of Conservation of Matter
• NH3 burns in oxygen to form NO & water
Law of Conservation of Matter
• C7H16 burns in oxygen to form carbon dioxide and water.
Calculations Based on Chemical Equations
• How many CO molecules are required to react with 25
MOLES of Fe2O3?
Calculations Based on Chemical Equations
• How many iron atoms can be produced by the reaction of
2.50E5 MOLES of iron (III) oxide with excess carbon
monoxide?
Calculations Based on Chemical Equations
• What mass of CO is required to react with 146 g of iron (III)
oxide?
Limiting Reactant Concept
• What is the maximum mass of sulfur dioxide that can be
produced by the reaction of 95.6 g of carbon disulfide with
110. g of oxygen?
CS2  3 O2  CO2  2 SO2
Limiting Reactant Concept
• What is the maximum mass of sulfur dioxide that can be
produced by the reaction of 95.6 g of carbon disulfide with
110. g of oxygen?
Percent Yields from Reactions
• Theoretical yield is calculated by assuming that
the reaction goes to completion.
– Determined from the limiting reactant calculation.
• Actual yield is the amount of a specified pure
product made in a given reaction.
– In the laboratory, this is the amount of product that is
formed in your beaker, after it is purified and dried.
• Percent yield indicates how much of the product is
obtained from a reaction.
actual yield
% yield =
 100%
theoretical yield
Percent Yields from Reactions
• A 10.0 g sample of ethanol, C2H5OH, was boiled with
excess acetic acid, CH3COOH, to produce 14.8 g of ethyl
acetate, CH3COOC2H5. What is the percent yield?
Balance this reaction
KClO3 +
C12H22O11
KCl + CO2 +
H 2O
Balance this reaction
CaSO4 + CH4 + CO2
CaCO3 + S + H2O