• Molar mass
is the mass of one mole of a
substance.
• Molar mass is numerically equal to atomic mass,
molecular mass, or formula mass. However …
• … the units of molar mass are g/mol.
• Examples:
1
atom
Na = 22.99
u
1
mol
Na = 22.99
g
1
formula unit
KCl = 74.56
u
The Mole and Molar Mass
1
mol
CO
2= 44.01
g
1
molecule
CO
2= 44.01
u
1
mol
KCl = 74.56
g
We can read formulas
in terms of moles of atoms or ions.
4
• Molecular mass
: sum of the masses of the
atoms represented in a molecular formula.
– Simply put: the mass of a molecule.
– Molecular mass is specifically for molecules.
– Ionic compounds don’t exist as molecules; for
them we use …
• Formula mass
: sum of the masses of the
atoms or ions present in a formula unit.
Molecular Masses and
Formula Masses
Determining the Formula/Molar
Mass of Ammonium Sulfate
The molar mass of ammonium sulfate is _______ g mol-1
The molar mass of acetic acid (CH3COOH) is ______ g mol-1
6
We can use these equalities to construct conversion factors:
Conversion Factors involving Mass, Moles, and
Number of Atoms/Molecules
1 mol CH3COOH = 6.022 ×1023CH3COOH molecules ≡ 60.05 g CH3COOH
60.05 g mol CH3COOH 1 60.05 mol g CH3COOH 6.022 × 1023 molec. mol (CH3COOH) 6.022 × 1023 molec. mol (CH3COOH) 1
The
mass percent composition
of a compound
refers to the mass proportion of the
constituent elements:
Mass Percent Composition
from Chemical Formulas
g element
% element = –––––––––––
×100
g compound
X
g element
X
% element = ––––––––––––––
100 g compound
OR …
#grams of the elementper 100 grams of the compound.
8
Percentage Composition of Butane
g/mol g/mol
• … is one method of determining empirical
formulas in the laboratory.
• This method is used primarily for simple organic
compounds (that contain carbon, hydrogen,
oxygen).
– The organic compound is burned in oxygen.
– The products of combustion (usually CO2and H2O) are weighed.
– The amount of each element is determined from the mass of products.
Elemental Analysis
תודוסי תזילנא
10
Elemental Analysis Setup
The sample is burned in a stream of oxygen gas, producing … … H2O, which is absorbed by MgClO4, and … … CO2, which is absorbed by NaOH.
Elemental Analysis
(cont’d) If our sample were CH3OH, every two molecules of CH3OH …… would give two molecules of CO2 …
… and four molecules of H2O.
12 Elemental analysis of butane (cooking gas) gave the following mass
composition:
C – 82.66%, H – 17.34%.
a) Determine the empirical formula of butane.
b) The molar mass of butane is 58.12 g/mol; what is the molecular formula?
Determine the empirical formula of phenol (a general disinfectant) from its elemental analysis:
C – 76.57% H – 6.43% H.
• A chemical equation is a shorthand description of a chemical reaction, using symbols and formulas to represent the elements and compounds involved.
Writing Chemical Equations
14
• Sometimes
additional
information
about the
reaction is
conveyed in
the equation.
Balancing chemical equations
תואוושמ ןוזיא
• A balanced equation must conform to the law of conservation of mass! הסמ ה רומ יש קוח
• To balance an equation we need to adjust the
stoichiometric coefficients םיירטמ ויכיוטס ם י מדק מ
Balance the following equations:
H2+ O2→ H2O C2H6+ O2→ CO2+ H2O
Fe + O2→ Fe2O3
16
• If an element is present in just one compound on each side of the equation, try balancing that element first.
• Balance any reactants or products that exist as the free element
last.
• In some reactions, certain groupings of atoms (such as
polyatomic ions) remain unchanged. In such cases, treat these groupings as a unit.
• At times, an equation can be balanced by first using a fractional coefficient(s). The fraction is then cleared by
Guidelines for Balancing
Chemical Equations
• A
stoichiometric factor
or mole ratio is a conversion
factor obtained from the stoichiometric coefficients in
a chemical equation.
ירטמויכיוטס
סחי
• In the equation: CO
(g)+ 2 H
2(g)→ CH
3OH
(l)Stoichiometric Equivalence
and Reaction Stoichiometry
1 mol CO
–––––––––
2 mol H
21 mol CO
–––––––––––––
1 mol CH
3OH
1 mol CH
3OH
–––––––––––––
2 mol H
22 mol H
2–––––––––
1 mol CO
1 mol CO reacted per 2 mol H2 2 mol H2 reacted per 1 mol CO 1 mol CO was consumed per 1 mol CH3OH that evolved1 mol CH3OH was formed for each 2 mol
of H2that reacted
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Outline of Simple Reaction Stoichiometry
Stoichiometric factor (or: mole ratio)
When 0.105 mol propane is burned in an
excess of oxygen, how many moles of oxygen
are consumed? The reaction is
C
3H
8+ 5 O
2→ 3 CO
2+ 4 H
2O
20
Outline of Stoichiometry Involving Mass
To our simple stoichiometry scheme … … we’ve added a conversion from mass at the beginning … … and a conversion to mass at the end.
Substances A and B may be two reactants,
two products, or reactant and product.
Think: If we are given moles of substance A initially, do we need to convert
dioxide with water; nitrogen monoxide is also produced.
How many grams of nitric acid are produced for every 5 kg of nitrogen dioxide that reacts?
22
Summary of Concepts
• Mass percent composition can be used to calculate the empirical formula.
– Additional information on the molecular weight of the material allows the calculation of the molecular formula
• A balanced equation must conform to the law of conservation of mass.
• Calculations involving reactions use stoichiometric factors based on stoichiometric coefficients in the balanced equation.