The Mole

```The Mole
Molar Mass


Molar mass relates moles to grams
The molar mass is the mass in grams of 1 mole of
an element or compound

Use the mass numbers from the periodic table, instead of
amu’s, the units are grams
Round all masses from the periodic table to the nearest tenth place

The chemical formula gives the ratio of atoms in a
compound

The chemical formula can also give the ratio of ions
to one another
Molar masses are computed in
the typical way
Volume-Mass Relationships of
Gases

Gay-Lussac’s law of combining volumes of
gases states that at constant temperature
and pressure, the volumes of gaseous
reactants and products can be expressed
as ratios of small whole numbers
 Example:
2 L of H2 reacts with 1 L of O2 to
produce 2L of H2O

volumes of gases at the same
temperature and pressure contain
equal numbers of molecules
 Also
indicates that volume is directly
proportional to the amount of gas, at
a given temperature and pressure

V=kn


law, one mole of any
gas will occupy the
same volume as any
other gas at the same
conditions
The standard molar
determined to be
22.41410 L (22.4L)
The Ideal Gas Law

The ideal gas law is the mathematical
relationship among pressure, volume,
temperature, and the number of moles of each
gas
1
V
P

V T
The ideal gas law is
derived by combining
Boyle’s law and
Charles’s law and the
law
V n
1
V  T  n
P
By adding a constant R we get:
1
V  R  T  n
P
Or PV= nRT

The constant R is known as the
ideal gas constant
 Its
value depend on the units
chosen
mRT
PV 
M

The ideal gas law can
be manipulated to
solve for molar mass
and density
 Given
that m = mass
and M = molar mass
 n=m/M and D=m/V
mRT
M
PV
MP
D
RT
Empirical formula
1.
2.
3.
4.
5.
Choose an arbitrary sample size (100g).
Convert masses to amounts in moles.
Write a formula.
Convert formula to small whole numbers.
Multiply all subscripts by a small whole
number to make the subscripts integral.
Example 3-5
Determining the Empirical and Molecular Formulas of
a Compound from Its Mass Percent Composition.
Dibutyl succinate is an insect repellent used against
household ants and roaches. Its composition is 62.58%
C, 9.63% H and 27.79% O. Its experimentally
determined molecular mass is 230 u. What are the
empirical and molecular formulas of dibutyl succinate?
Step 1: Determine the mass of each element in a 100g
sample.
C 62.58 g
H 9.63 g
O 27.79 g
Example
3-5
Step 2: Convert masses to amounts in moles.
1 mol C
 5.210 mol C
12.011 g C
1 mol H
nH  9.63 g H 
 9.55 mol H
1.008 g H
1 mol O
nO  27.79 g O 
 1.737 mol O
15.999 g O
nC  62.58 g C 
Step 3: Write a tentative formula.
C5.21H9.55O1.74
Step 4: Convert to small whole numbers. C2.99H5.49O
Slide 17 of 37
Prentice-Hall &copy; 2002
General Chemistry:
Chapter 3
Example
3-5
Step 5: Convert to a small whole number ratio.
Multiply x2 to get C5.98H10.98O2
The empirical formula is C6H11O2
Step 6: Determine the molecular formula.
Empirical formula mass is 115 u.
Molecular formula mass is 230 u.
The molecular formula is C12H22O4
Slide 18 of 37
Prentice-Hall &copy; 2002
General Chemistry:
Chapter 3
Concentration &amp; Molarity

The ratio of the solute to the solvent is the solution
concentration



Molarity is the moles of solute per liter of solution
It is abbreviated with an M
5.0 M HCl is read as a 5 molar solution of HCl
molesofsolute
molarity 
litersofsolution
Molality is the concentration of a solution
expressed in moles of solute per kilogram
of solvent
 Symbol is m

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