Chapter 11
 Substances react according to definite ratios
of numbers of particles (atoms, ions, formula
units or molecules).
 Balanced chemical equations tell us what the
ratios of these particles is.
 If we want to know how many of these
particles we have, we can find it by measuring
the mass .
 Scientists have found that the atomic mass of
an element contains 6.02 X 1023 atoms of that
element.
 This number is known as Avogadro’s
number.
 This number is also equivalent to 1 mole of
particles.
 The mole is the SI base unit for the amount of
substance.
 It is a quantity, just like a dozen, a pair, or a
gross!!
 It is defined as the number of particles in exactly
12 g of pure carbon-12.
 When dealing with moles, the simplest unit of
any substance is called a representative particle.
 The particle may be an atom, a molecule, a
formula unit, or an ion.
 How many molecules are in 2.25 moles of
Br2?
 Bromine is an element that consists of
diatomic molecules. One mole of bromine
contains 6.02 X 1023 bromine molecules.
 Using dimensional analysis,
2.25 molBromine x 6.02 X 1023 molecules Bromine
1 molBromine
= 1.35 X 1024 molecules Br2
1. Calculate the number of molecules in 15.7
moles of carbon dioxide.
2. Calculate the number of molecules in 0.0544
moles of H2O.
3. Calculate the number of moles in 9.22 X 1023
atoms of iron.
4. Calculate the number of moles of sucrose in
a sample that contains 2.05 X 1022 sucrose
molecules.
 One mole of a monatomic element consists
6.02 X 1023 atoms of that element.
 The mass of a mole of any substance is called
the molar mass of the substance.
 The molar mass of a monoatomic element is
numerically equal to the atomic mass of
element, but expressed in grams.
1. Calculate the mass of 6.89 mol of antimony.
2. A chemist needs 0.0700 mol selenium for a
reaction. What mass of selenium should the
chemist use?
3. A sample of sulfur has a mass of 223 g. How
many moles of sulfur are in the sample?
4. Calculate the number of atoms in 2.00g of
platinum.
5. How many sulfur atoms are in a metric ton
(1.00 106) of sulfur?
 Remember that a mole of a compound
contains as many moles of each element as
are indicated by the subscripts in the formula
of the compound.
 For example, a mole of ammonia (NH3)
contains one mole of nitrogen and three
moles of hydrogen.
 The molar mass of a compound is the mass of
a mole of the representative particles of the
compound.
 To find the molar mass, add up the molar
masses of all the atoms in the representative
particle.
 For example, in NH3 you would add up the
molar mass of nitrogen, and three times the
molar mass of hydrogen.
 Mmammonia = 14.01 + 3 (1.01)
= 14.01 + 3.03
= 17.04 g
1. Calculate the number of moles in 17.2 g of
benzene (C6H6).
2. Calculate the number of moles in 350.0 g of
potassium chlorate.
3. Determine the mass of 0.187 mol of tin (II)
sulfate (SnSO4).
 Remember that every chemical compound
has a definite composition—a composition
that is always the same wherever that
compound is found.
 The composition of a compound is usually
stated as the percent by mass of each
element in the compound.
 The percent composition of an element can
be found in the following way.
% mass = mass of element in compound X 100%
Molar mass of compound
 Determine the percent composition of
calcium chloride (CaCl2).
 %Ca = 40.078 g Ca X 100% = 36.112% Ca
110.98 g CaCl2
 %Cl = 70.906 gCl X 100% = 63.888% Cl
110.984 g CaCl2
 The empirical formula is the simplest whole-
number ratio of atoms of elements in the
compound.
 The percent composition of an unknown
compound is found to be 38.43% Mn, 16.80%
C and 44.77% O. Determine the compound
the compound’s empirical formula.
1. The composition of acetic acid is 40.00 %
carbon, 6.71% hydrogen, 53.29 % oxygen.
Calculate the empirical formula for acetic
acid.
2. Calculate the empirical formula for a
compound whose analysis is 74.97 %
aluminum and 25.03% carbon.
 For many compounds, the empirical formula
is not the true formula.
 A molecular formula tells the exact number
of atoms of each element in a molecule or
formula unit.
 The molecular formula is always a wholenumber multiple of the empirical formula.
 To find, you must know the molar mass of the
compound and its empirical formula.
 Maleic acid is a compound that is widely used
in plastics and textiles. The composition of
maleic acid is 41.39% carbon, 3.47% hydrogen
and 55.14% oxygen. Its molar mass is 116.1
g/mol. Calculate the molecular formula.
1. Ricinine is one of the poisonous compounds
found in the castor plant. The composition of
ricinine is 58.54% carbon, 4.91% hydrogen,
17,06% nitrogen and 19.49% oxygen.
Ricinine’s molar mass is 164.16 g/mol. Find its
molecular formula.
2. A compound of phosphorous and sulfur
contains 27.87% phosphorous and 72.13%
sulfur. The molar mass of the compound is
222.3 g/mol. Calculate its molecular formula.
 Many compounds, particularly ionic
compounds, include a specific number of
water molecules into their crystals when they
crystallize from aqueous solution.
 Compounds that include water in their crystal
structures are called hydrates.
 The formula is written by adding the formula
for water to the formula for the compound,
and the number of waters is shown with a
coefficient.
 A hydrate of aluminum bromide is composed
of 71.16% AlBr3 and 28.84% H2O. What is the
formula of the hydrate?
1. Cerium (III) iodide (CeI3) occurs as a hydrate
with the composition 76.3% CeI3 and 23.7%
H2O. Calculate the formula of the hydrate.
2. Lead (II) acetate (Pb(C2H3O2)2) exists as a
crystalline hydrate that is 14.25% water by
mass. What is the molecular formula for this
hydrate?