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Ch. 11
The Mole
11.1 Measuring Matter
 Mole- SI base unit used to
measure the amount of a
substance
 Equal to the number of
representative particles (carbon
atoms) in exactly 12 grams of
carbon-12
 Representative particle :
 Elements –atom
 Covalent –molecules
 Ionic - formula units
Representative particle
Elements
Atoms
Molecules
Compounds
Ionic
Compound
Formula
Unit
Covalent
Compound
Molecules
Avogadro’s Number
 Avogadro’s Number
= 6.022 X 1023
 Is a very large
number because it is
used to count
extremely small
particles.
Conversion of a Mole
to a Particle
 1 mole = 6.02 x 1023
 # of moles x 6.02 x 1023
representative particles = # of
representative particles
 Ex: How many molecules are
in 3.5 moles of sucrose?
 3.5 moles sucrose x 6.02 x 1023 molecules
=
1 mole sucrose
 = 2.11 x 1024 molecules of sucrose
Conversion of a Particle to
a Mole
 Reverse conversion factor to
solve for # of moles
 Ex: How many moles are in 4.50 x
1024 atoms of zinc?
 4.50 x 1024 atoms Zn x
= 7.48 mol Zn
1 mol Zn
6.02 x 1023 atoms Zn
STOP! YOUR TURN!
 Practice Problems 11.1
11.2 Mass and the Mole
 Just as a dozen bricks and a
dozen feathers don’t have the
same mass, moles of different
substances also have different
masses.
 Molar mass- mass in grams of
one mole of any pure substance
 The molar mass of any element
is numerically equal to its atomic
mass and has the units g/mol
Using Molar Mass
 Ex: What is the mass in grams of
0.0450 moles of chromium?
 Moles Cr x grams Cr = grams Cr
1 mole Cr
 0.0450 mol Cr x 52.00 g Cr = 2.34 g Cr
1 mol Cr
Mols
Mass
Particles
(atoms, molecules, formula unit)
Converting Mass to Atoms
and Atoms to Mass
 Ex: How many atoms of gold are in a
pure nugget having a mass of 25.0 g?
 Known:
 Mass = 25.0 g Au
 Molar mass Au = 196.97 g/mol Au
 Unknown:
 Number of atoms = ? Atoms Au
 Mass Au x 1 mole Au = moles Au
# g Au
 25.0 g Au x 1 mol Au = ? mol Au
196.67 g Au
= 0.127 mol Au
Half way there!!
 Moles Au x 6.02 x 1023 atoms Au =
1 mole Au
 0.127 mol Au x 6.02 x 1023 atoms Au
1 mol Au
= 7.65 x 1022 atoms Au
 Mass must always be converted
to moles before being converted
to atoms, and atoms must be
converted moles before
calculating their mass.
11.3 Moles of Compounds
 Chemical formula indicates types
of atoms and number of each in
one unit of the compound
 Ex: CCl2F2
 Carbon = one atom
 Chlorine = 2 atoms
 Fluorine = 2 atoms
 Ratio of carbon to chlorine to
fluorine is 1 : 2 : 2
Conversions with
Chemical Formulas
 How many moles of fluorine atoms
are in 5.50 moles of freon
(CCl2F2)?
 5.50 mol CCl2F2 x 2 mol F atoms = 11.0 mol F atoms
1 mol CCl2F2
Molar Mass of Compounds
 Mass of a mole of a compound
equals the sum of the masses of
every particle that makes up the
compound.
 Suppose you want to determine
the molar mass of potassium
chromate (K2CrO4)
 # moles x molar mass = # grams
 2.000 mol K x 39.10 g K = 78.20 g K
1 mol K
1.000 mol Cr x 52.00 g Cr = 52.00 g Cr
1 mol Cr
4.000 mol O x 16.00 g O = 64.00 g O
1 mol O
 Molar mass = 78.20 g K
52.00 g Cr
+ 64.00 g O
194.20 g K2CrO4
Converting Moles of a
Compound to Mass
 Step 1: Calculate molar mass of
the compound.
 Step 2: Convert moles to grams
using the molar mass as a
conversion factor.
Converting Moles to Mass
 Convert 2.50 mol CHCl3 to mass in
grams.
 Step one: Calculate the number of grams
in one mole of CHCl3 = 119.35 grams
 Step up problem as before
 What’s given What you want
What you want to get rid of
 2.5 mol CHCl3 119.35 grams of CHCl3
1 mol of CHCl3
298.38 grams of CHCl
Converting Mass of a
Compound to Moles
 Use inverse of mole to mass conversion
factor
Converting Mass of a
Compound to Number of
Particles
 Step 1: Convert given mass to
moles by using the molar mass
as a conversion factor.
 Step 2: Convert moles to
number of representative
particles by multiplying by
Avogadro’s number.
11.4 Empirical and
Molecular Formulas
 Percent Composition- percent by mass of
each element in a compound
 % composition = mass of element x 100
mass of compound
Percent Composition from
the Chemical Formula
 Percent composition is always
the same, regardless of the size
of the sample
 To determine percent
composition, assume a sample
size of one mole
What is the percent
composition of water?
Percent Hydrogen:
 2.02 g H
x 100 = 11.2% H
18.02 g H2O
Percent Oxygen:
16.00 g O x 100 = 88.80% O
18.02 g H2O
STOP! YOUR TURN!
 Page 331, #45
Empirical Formula
 Empirical Formula- formula with
the smallest whole number mole
ratio of the elements
 Which of these do you think is
the empirical formula?
HO or H2O2 ?
 Empirical formula may be
different from molecular formula
 Ex: hydrogen peroxide
Empirical formula: HO
Molecular formula: H2O2
Finding the Empirical
Formula
 If percent composition is given,
assume a 100 g sample, so
change % to grams
 Ex: percent composition is
40.05% S and 59.95% O
 100.0 g of the sample is 40.05
g S and 59.95 g O
 Convert mass of each element to number
of moles
 40.05 g S x 1 mol S = 1.249 mol S
32.07 g S
 59.95 g O x 1 mol O = 3.747 mol O
16.00 g O
 Ratio of S atoms to O atoms is
1.249 : 3.747
 Must convert to whole numbers
 Since 1.249 is smallest, divide
both numbers by that value
Continued…
 1.249 mol S = 1 mol S
1.249
 3.747 mol O = 3 mol O
1.249
 Ratio of S atoms to O atoms is 1 : 3
 Empirical Formula = SO3
STOP! YOUR TURN!
 # 46 and 47 page 333
Molecular Formula
 Molecular Formula- specifies the
actual number of atoms of each
element in one molecule or
formula unit of the substance
 Molecular formula = (empirical
formula) n
 n is the factor by which the
subscripts in the empirical
formula must be multiplied to
obtain molecular formula
Determining Molecular
Formula
 Empirical formula: C2H3O2
 Molar Mass C2H3O2 = 59.04 g C2H3O2
(calculated value)
 Molar Mass succinic acid = 118.1 g
(given value)
 n = molar mass succinic acid
molar mass C2H3O2 (empirical formula)
 n = 118.1 g = 2.00
59.04 g
 (C2H3O2) 2 = C4H6O4
STOP! YOUR TURN!
 # 51 and 52
 Empirical Formula = HO
 Given the molar mass is 34.014g/mol.
What is the MOLECULAR FORMULA?
Class Discussion
 Why do one mol of sulfur not the same
as one mol of hydrogen?
 CROSS OFF 11.5
11.5 The Formula for a
Hydrate
 Hydrate- a compound with a
specific number of water
molecules bound to its atoms
Analyzing a Hydrate
 In order to analyze a hydrate,
water must be removed (usually
from heating)
 Substance remaining after
heating is “anhydrous” (meaning
without water)