Chemical Quantities
Standards
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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.
Objectives
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Know the meaning of the mole
Know how to convert between moles, grams,
and number or particles
Know how to calculate the molar mass of a
compound
Know how to calculate percent composition
Know how to determine empirical, molecular
and hydrate formulas
Identity of some basic
“particles” in chemistry
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Element
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Atom
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Molecule…diatomic elements
Compounds
Covalent vs Ionic
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Covalent Compounds
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Are composed of molecules
Compounds
Covalent vs Ionic
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Ionic Compounds
 Are composed of positive and negative ions; the
representative particle is called a formula unit
 Formula unit: the simplest ratio of ions
represented in an ionic compound
Mole
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Defn: a counting reference. A quantity chosen by chemists to
represent the amount of a substance.
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It is the number of representative particles in exactly 12 grams of pure
carbon-12
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The amount of a substance which contains “6.022 x 1023 particles”
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Particles can be: Atoms, molecules, formula units, ions, etc.
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6.022 x 1023 is known as Avogadro’s Number
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1 mole = the atomic mass of that element in grams...from the PT
Question
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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 grams of fluorine or 78.96 g of Se because they
each contain 1 mole of particles
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1 mole of He = ________g He
58.69 g Ni = ________mole Ni
Atomic Mass
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Atomic Mass: the weighted average mass of
the isotopes of an element…the value on the PT
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What is the atomic mass of hydrogen?
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1.00794 g
What is the atomic mass of Oxygen?
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15.999 g
Avogadro's Number
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Avogadro's Number is 6.022x1023 which is
the number of representative particles in
a mole
6.022x1023 “particles” = 1 mole
Mole Conversions
MOLES ↔ GRAMS
MOLES ↔ ATOMS
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Determine the “given” and the “get”
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Mini road map
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Set up equation with conversion factor(s)
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Cancel the units and calculate.
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Does your answer make sense?
Example
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2.00 moles Boron = how many grams Boron?
Example
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How many moles of calcium are there in
23.20 grams of calcium?
Example
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How many atoms are there in 5.25 moles of
nitrogen gas?
Put the Moles in the Middle!
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GRAMS
↔ MOLES
↔ ATOMS
“MOLES IN THE MIDDLE,”
you gotta go through the MOLE!
Generic Mole Conversion Set-Up
Example
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How many atoms are there in 27.35 g
of aluminum?
Example
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How many grams are there in 6.7 x 1024
molecules of HF?
Molar Mass
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Defn: the sum of the atomic masses of all the
elements in a compound/element
Calculating Molar Mass
1)
Determine the atomic mass of each element
in the compound
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2)
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The atomic mass is found on the periodic table
Add all the atomic masses together, this is
your molar mass
EX. What is the molar mass of water?
H2O: 2 H = 2 ( 1.00794 g)
1 O = 15.9994 g/mol
2 ( 1.00794 g) + 15.9994 g= 18.0153g/mol
Question
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What is the molar mass of calcium
sulfate?
Percent Composition
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The percent by mass of each element in a compound
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To determine the percent composition of a compound:
1)
2)
3)
4)
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Determine the molar mass of the compound
Determine the molar mass of the element
Determine the percent of each element in the compound,
using the % composition formula
Check to make sure percentages add up to 100%
% El = (# atoms of El) (atomic mass El) x 100
molar mass of compound
Question
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What is the percent composition of H2O?
% El = (# atoms of El) (atomic mass El) x 100
molar mass of compound
% H = (2) ( 1.00794 g) x 100 = 11.2% H
18.0153 g
% O =?
100-11.2=88.8%
Examples
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Calculate the percent composition of the following compounds:
C2H6
CO2
NaHSO4
CO2
NaHSO4
Empirical Formulas
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Defn: the formula with the smallest whole number mole ratio of elements in a
compound
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If given percent composition, then:
1)
Assume a 100 gram sample
2)
Find the number of moles of each element
3)
Divide each mole number by the smallest mole number
4)
Write the formula
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If you come up with a decimal number, multiply the decimal number by a whole
number to get a whole number
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Then multiply all other ratios by that same whole number
Example
Given: 94.11% O and 5.89% H
Assume 100 g sample→
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Example
Given: 52.9% C and 47.1% O
Assume a 100 g sample→
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Empirical Formulas
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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
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If you come up with a decimal number, multiply the decimal number by
a whole number to get a whole number
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Then multiply all other ratios by that same whole number
Question
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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
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Defn: The actual number of atoms of each element in one molecule or formula
unit of the substance
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You calculate a molecular formula from an empirical formula
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To find the molecular formula:
1)
find molar mass of empirical formula
2)
Divide molar mass (given) by empirical formula mass to get an “integer”
Molar mass of compound = N (an integer)
empirical formula mass
3)
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Multiply empirical formula by the “integer”
(Molecular Formula) = (Empirical Formula)N…
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N distributes to all the subscripts
Example
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The molar mass of a compound is 42 g/mol. Its
empirical formula is CH2. What is the molecular
formula for this compound?
Example
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What is the molecular formula for a compound with an empirical
formula of OH and a molar mass of 34 g/mol?
Hydrate Formulas
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Defn:a solid which crystallizes with water
molecules bonded to the compound in the
crystalline lattice.
BaCl2 •2H2O → BaCl2(s) + 2 H2O (g)
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BaCl2 •x H2O → BaCl2(s) + x H2O (g)
Name of the hydrous compound:
To Name a hydrous compound:
Name of the ionic compound + prefix hydrate…
(the prefix corresponds to the coefficient in front of the water)
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Prefixes:
1-mono
2- di
3-tri
4-tetra
5-penta
6-hexa
7-hepta
8-octa
9-nona
10-deca
Finding a Hydrate Formula
1)
2)
Find the number of moles for each compound
Divide each mol number by the smallest mol
number
3)
Write the formula
4)
Name the compound
Example
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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 solid residue remain. Find the
hydrate formula.
The End