HP
Chapter 6: Chemical Composition
TOPICS:
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The Mole and its Conversions
Molar Mass
Percent Composition
Empirical Formulas
Molecular Formulas
Hydrate Formulas
Standards Addressed:
Conservation of Matter and Stoichiometry
3b. Students know the quantity of one mole is set by defining one mole of carbon-12 atoms to
have a mass of exactly 12 grams.
3c. Students know one mole equals 6.02 x 1023 particles (atoms or molecules).
3d. Students know how to determine the molar mass of a molecule from its chemical formula
and a table of atomic masses and how to convert the mass of a molecular substance to
moles, number of particles, or volume of gas at standard temperature and pressure.
Anticipatory Set: Remember Dimensional Analysis? Here it again, in a chemistry context.
Objective: By the end of the lesson, the student will:
 Know the meaning of the mole
 Know how to convert between moles, grams, and number of particles
 Know how to calculate the molar mass of a compound
 Know how to calculate percent composition
 Know how to determine empirical formulas, molecular formulas and hydrate formulas
Purpose: Future purpose.
 We will use this information to solve stoichiometry problems. You will be tested on this in the Spring. You need
this information for AP Chemistry. This is some of the basic chemistry information that you will need in your
college chemistry courses.
Teacher Input
Identity of some basic “particles” in chemistry
Elements
 Atoms
 Molecules
copper atoms
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Compounds
 Covalent compounds
o
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Ionic Compounds
o
crystalline lattice structure
Mole (mol): a counting reference. The mole is used to measure the amount of substance
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It is the number of representative particles in exactly 12 grams of pure carbon-12
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Where do you find the atomic mass for an element?
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12 grams of carbon contains the same # of atoms as 19 g F or 78.96 g Se
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1 mole of He = ________g He
58.69 g Ni = ___________ mole Ni
Atomic Mass:
Examples: What is the atomic mass of hydrogen ? ___________
What is the atomic mass of oxygen? ___________
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Avogadro’s Number
MOLE CONVERSIONS
MOLES ↔ GRAMS
and
MOLES ↔ ATOMS
1.
2.
3.
4.
5.
Examples:
2.00 moles Boron = how many grams Boron?
How many moles of calcium are there in 23.20 grams of calcium?
How many atoms are there in 5.25 moles of nitrogen gas?
GRAMS ↔ MOLES ↔ ATOMS “MOLES IN THE MIDDLE”, you gotta go through the MOLE!
How many atoms are there in 27.35 g of aluminum?
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How many grams are there in 6.7 x 1024 molecules of HF?
Molar Mass: the sum of the atomic mass of all the elements in a compound/element in units of g/mol
To Calculate Molar Mass of a compound:
1.
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2.
Examples: What is the molar mass of water?
H2O:
What is the molar mass of Calcium sulfate?
Percent Composition: the relative amount, by mass, of each element in a compound
To determine percent composition:
1.
2.
3.
4.
% El = ( # atoms of El) (atomic mass of El)
molar mass of compound
x 100
Example: What is the percent composition of H2O? → % H =
%O=
4
Calculate the percent composition of the following compounds:
 C2H6
 CO2

NaHSO4
EMPIRICAL FORMULAS
Empirical formula (e.f.): the formula with the smallest whole number mole ratio of the elements
If given percent composition, then:
1.
2.
3.
4.
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Examples:
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Given: 94.11% O and 5.89% H, determine the empirical formula.
Assume 100g →
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Given: 52.9% C and 47.1% O, determine the empirical formula.
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If given the mass of a compound and the mass of the individual elements, then:
1. Determine how many grams of each element is present
2. find the number of moles of each element
3. divide each mole number by the smallest mole number
4. write the formula
 if you come up with a decimal number, then multiply the decimal number by a
whole number to get a whole number
 then multiply all other ratios by that same whole number
Example:
Analysis of 20.0 g of a compound containing only calcium and bromine indicates that 4.00 g of
calcium are present. What is the empirical formula of the compound formed?
MOLECULAR FORMULAS
Molecular Formula: The actual number of atoms of each element in one molecule or formula
unit of the substance.
 If the molecular formula is different than the empirical formula, the molecular formula
will always be a simple multiple of the empirical formula.
You calculate the molecular formula from the empirical formula.
To find the molecular formula:
1.
2.
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_____Molar mass of Compound____ = N (an integer)
Empirical formula mass
3.
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(Molecular Formula) = (Empirical Formula)N…N distributes to all the subscripts
Examples:
The molar mass of a compound is 42 g/mol. Its empirical formula is CH2. What is the
molecular formula for this compound?
What is the molecular formula for a compound with an empirical formula of OH and a
molar mass of 34 g/mol?
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HYDRATE FORMULAS
Hydrate: a solid which crystallizes with water molecules trapped in the crystalline lattice structure
BaCl2 ∙ 2H2O → BaCl2(s) + 2 H2O (g)
BaCl2 ∙ x H2O → BaCl2(s) + x H2O (g)
Name of the above hydrous compound:
To Name the hydrous compound:
Name of the ionic compound + prefix hydrate…(the prefix corresponds to the coefficient in front of the water)
Prefixes:
1-mono
2- di
3-tri
4-tetra
5-penta
6-hexa
7-hepta
8-octa
9-nona
10-deca
To find the formula for a hydrate: (similar to finding an empirical formula)
1.
2.
3.
4.
Example:
A hydrate of CuSO4 is heated to drive off the water of crystallization. When 10.0 g of the
hydrate is heated, 6.39 grams of the solid residue remain. Determine the formula for the
hydrated compound.
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