Download Chemical Formulas and Composition Stoichiometry

Chemical Formulas and
Composition Stoichiometry
Chapter 2
Stoichiometry
• Describes the ___________ relationships
among elements in compounds
(composition stoichiometry) and among
substances as they undergo chemical
changes (reaction stoichiometry).
Dalton’s Atomic Theory
• An element is composed of extremely small,
indivisible particles called _____.
• All atoms of a given element have ______
properties that are ______.
• Atoms cannot be _____, ______, or ______ into
atoms of another element.
• ______ are formed when atoms of different
elements combine with one another in small
whole-number ratios.
• The relative _____ and ____ of atoms are constant
in a given compound.
Atoms and Molecules
• We know what atoms are and the fundamental
particles that compose them.
• _______ is the smallest particle of an element
or compound that can have a stable, ______
existence.
– This means that if a molecule contains only one
type of atom it is also an element.
Atoms and Molecules
• Monatomic – contains only ___ atom
– Na and He For these elements, an atom and a molecule are
the same.
• Diatomic – molecules that contain only __ atoms
– H2, O2, and all halogens
• Polyatomic – molecules than contain ______ atoms
– P4, S8, and C60
Allotropes – different _____ (geometry or number of
atoms) of the same element
Compounds
• Compounds are substances that contain two
or more elements in fixed proportions
– Compounds containing only covalent bonds
• The atoms in a compound _________ electrons.
• These compounds are also called molecules since
they exist as individual particles.
– Compounds containing ionic bonds
• These compounds are held together by electrostatic
interactions.
• These are not molecules since they do not exist as
individual particles.
Covalent Compounds
• The chemical formula represents the ____ present
and the ______ in which the atoms of the elements
occur.
– Table 2.2 Show a few models to illustrate that all atoms
are held together by covalent bonds.
• H2O and C4H10 (models)
• Notice that the formula is not necessarily the simplest ratio
• _________ compounds are largely composed of
C-C,C-H, and C-O bonds.
– Ethanol and benzene
Covalent Compounds
• Inorganic compounds largely do not contain _____
and/or ____ bonds.
– H2O and NH3
• Law of Definite Proportions – different pure samples
of a compound always contain the same elements in
the same proportion by mass
– _______ of atoms can also affect the properties of
compounds even if they have the same chemical formula.
Demo: John Cullen’s demonstration
Compounds Containing Ionic
Bonds
• Held together by __________ between ions
• They do not exist as individual units
– Collection of a large number of ions
– The formula represents the ______ of ions in the
compound
• Each positively-charged species is surrounded by
negatively-charged species (and vice-versa)
– DEMO: NaCl structure
Writing Formulas and Names for
Ionic Compounds (Table 2-3)
• Positive and negative ions will combine in such
a way to make the ionic compound _____.
• Binary Ionic Compounds
– You already know how to write the formulas.
• Write the formula for calcium bromide and lithium sulfide
– Naming a binary compound from the formula
• Cation – name of the metal
• Anion – drop the last portion of the name and add ‘ide’
• Name AlBr3 and BeI2
Common Ions
More Polyatomic Ions
Writing Formulas and Names for
Ionic Compounds (Table 2-3)
• Ionic compounds containing a cation that
can have multiple charges
– The charge on the cation has to be specified
– What is the charge on FeCl3 and FeCl2?
• In most cases, the charge can be determined by the
negatively-charged ion
• Write the names for these compounds?
– Write the formulas for copper(II) sulfide and
iron(III) oxide.
Writing Formulas and Names for
Ionic Compounds (Table 2-3)
• Ionic compounds containing polyatomic
ions
• The atoms in polyatomic ions are held
together by _________.
– Write the Lewis structure for SO32-
• The polyatomic ions and the oppositelycharged ions are held together in the solid
by _________
Writing Formulas and Names for
Ionic Compounds (Table 2-3)
• Write the names for (NH4)2S and K2(SO4).
– Notice that the charges are fixed and given on
Table 2-3 for the polyatomic ions.
• Write the formulas for magnesium nitrate
and lithium carbonate.
– If more than one polyatomic ion is needed
parentheses are placed around it
Atomic Weight
• The atomic weight is based on the atomic
mass unit.
– One amu is _____ the mass of a carbon-12
atom
• The atomic weight, which is given on the
periodic table, is a _______ of all the
naturally occurring isotopes of that element
The Mole
• A mole describes a ________ of objects,
particles, or atoms. This is similar to other
measurements that describe a particular
quantity.
– 1 dozen = 12 items
– Gross = 144 items
– 1 mole = _____________ (much larger)
• Avogardo’s number
The Mole
• A mole is the amount of substance that contains as
many entities as there are atoms in 12.0 grams of
carbon-12.
• One mole of H2O would contain 6.02  1023
molecules of H2O.
• The mass of one mole of atoms of an element is
equal to its atomic weight (periodic table) in
grams.
– This is called the molar mass (units ________).
The Mole
• Iron (Fe)
55 .847 g Fe
6 .02  10 23 Fe atoms
6 .02  10 23 atoms
and
and
1 mole Fe
55 .847 g Fe
1 mole Fe atoms
• These conversion factors will be used extensively!!
• Calculate the mass of a single Mg atom.
• Calculate the number of atoms in 1.40  10-15 moles
of Cu.
• Calculate the number of moles in 85.3 grams of Al.
Molar Mass
• The ________ of a substance is the sum of the
atomic weights of all the atoms in the formula.
Expressed in amu.
• _________ can be used if the compound is
composed of molecules.
• Molar mass is _________ to the formula weight of
a compound. Expressed in grams
– Indicates how many grams in ______ of the compound
Molar Mass
• This molar mass of a substance will contain
6.02  1023 formula units
molar mass H 2 O
1 mole H 2 O
6 .02  10 23 formula units H 2 O
FW H 2 O 
and
and
1 mole H 2 O
6 .02  10 23 formula units H 2 O
molar mass H 2 O
• What is the molar mass of Ca(NO3)2?
• Calculate the number of C3H8 molecules in
75.4 grams of C3H8
• Calculate the number of millimoles in 0.234
g of oxalic acid (CHOO)2
More Problems
• The chemical formula indicates the relative
number of ______ and _____ in a compound.
– Shown by the ________
C3H8
• How many formula units in 2.5 moles of CaCl2?
How many Ca2+ and Cl- ions in 2.5 moles of
CaCl2?
• Calculate the number of oxygen atoms in 54.2
grams of sucrose, C6H12O6.
• Calculate the number of moles of hydrogen in
54.2 grams of sucrose.
Percent Composition
• The mass percent of each element present in
a compound is simply determined by
dividing its mass by the molar mass of the
compound multiplied by 100%.
• Let’s do H2O. What is the percent
composition of oxygen and hydrogen?
• What is the percent composition of carbon
and hydrogen in C3H8?
Percent Composition
• The percentages must add up to _____.
Any pure compound will contain the same
percentages of each element (____ __
_____ ________).
• One more – What is the percent
composition of Fe in Fe2(SO4)3?
Determining Empirical Formulas from
Percent Composition
• Empirical formula (or simplest formula) is the
__________ ratio of elements in a compound.
• Molecular formula is the ________ of atoms
present in a compound containing covalent bonds.
The empirical formula and molecular formula are not
necessarily the same.
– Benzene (show structure) Molecular vs. empirical
– Hydrogen peroxide (show structure) Molecular vs.
empirical
– Sucrose
Determining Empirical Formulas from
Percent Composition
• A compound contains 24.74% K, 34.76% Mn, and
40.50% O. What is the empirical formula of this
compound?
– Step 1: Consider how many grams of each element
would be present in 100.0 grams.
– Step 2: Convert to moles for each element.
– Step 3: Obtain smallest whole-number ratio by
dividing by the smallest mole number.
– Step 4: If necessary, convert fraction to whole numbers
by multiplying an integer.
Determining Empirical Formulas from
Percent Composition
• A compound contains 7.537% H, 26.185%
N, and 66.278% Cl. What is the empirical
formula?
• A sample of a compound contains 6.541
grams of Co and 2.369 grams of O. Find
the simplest formula.
Combustions Train – Analysis of
Hydrocarbons
• Combustion trains are used to perform quantitative
carbon-hydrogen analysis.
• A known mass is burned in a stream of oxygen.
– Carbon is converted to CO2
– Hydrogen is converted to H2
Weight increase in the absorber region is measured.
Combustions Train – Analysis of
Hydrocarbons
• Determine the mass of carbon in the compound
– Use the conversion factor
12 . 01 g C
44 . 01 g CO 2
• Determine the mass of oxygen in the compound
– Use the conversion factor
2 .016 g H
18 .02 g H 2 O
Why do we use these conversion factors?
Which units will cancel?
Combustions Train – Analysis of
Hydrocarbons
• A 0.2815-gram sample of pure hydrocarbon
was burned in a combustion train to produce
0.4362 grams of H2O and 0.8527 grams of
CO2. Determine the masses of C and H in
the sample and the percentages of these
elements in this hydrocarbon.
I’ll give you one on your chapter problems.
Combustions Train – Analysis of
Hydrocarbons
• Compounds that contain C, H, and O.
• A 0.3615-gram compound containing on C,
H, and O was burned in a combustion train
to produce 0.8219 grams of CO2 and 0.3364
grams of H2O. Determine the masses of C,
H, and O in the sample and the percentages
of these elements in this hydrocarbon.
Determining Molecular Formulas
• Percent composition only yields empirical
formulas.
– Determine the empirical formulas for the last two
problems.
• In order to determine the molecular formula,
both the _______ and the _________ must be
known.
Determining Molecular Formulas
• The molecular formula is an integer multiple of
the simplest formula.
– C6H6 and C6H12O6
– Molecular formula = n  simplest formula
– It follows that;
• Molecular weight = n  simplest formula weight (rearrange to
solve for n)
• The molecular weight of the compound in the first
problem was determined to be 58.123 amu. What
is the molecular formula?
Determining Molecular Formulas
• The molecular weight of the compound in
the second problem was determined to be
174.240 amu. What is the molecular
formula?
Have fun!!!
More Fun with Conversions
• What mass of ammonium phosphate,
(NH4)3PO4 would contain exactly 15.00
grams of N?
• What mass of sucrose, C6H12O6, would
contain the same mass of oxygen as is
contained in 25.1 grams of ammonium
phosphate?
Solid Hydrates
• Solid hydrates are crystalline solids that contain
_______ which can be generally removed by
heating.
– DEMO: CuSO45H2O The water can be removed by
heating.
– Every mole of CuSO45H2O contains _____ of water.
• An unidentified hydrate of magnesium sulfate is
heated, MgSO4xH2O. The mass before heating
was 78.30 grams. The new mass after heating is
38.24 grams. What is the formula of the hydrate?
Purity of Samples
• Samples are never 100% pure. Percent purity is
used to specify the purity of a particular sample.
mass of pure subs tan ce
% purity 
 100 %
mass of sample
• In many cases, 100 grams of sample can be used
for convenience.
• A bottle of Na3PO4 is 98.3% pure. What are the
masses of Na3PO4 and impurity in 251 grams of
sample?
• Calculate the number of moles of C2H4Cl2 in 58.2
g of 92.1% pure C2H4Cl2.
Solving Problems
Let’s Do Some More Problems