How you measure how much?
You can measure mass,
„ or volume,
„ or you can count pieces.
„ We measure mass in grams.
„ We measure volume in liters.
„
Chapter 7
Chemical Quantities
„
Moles
Representative particles
Defined as the number of carbon
atoms in exactly 12 grams of carboncarbon12.
23 particles.
„ 1 mole is 6.02 x 10
„ Treat it like a very large dozen
23 is called Avagadro’
„ 6.02 x 10
Avagadro’s
number.
The smallest pieces of a substance.
„ For a molecular compound it is a
molecule.
„ For an ionic compound it is a formula
unit.
„ For an element it is an atom.
„
„
Types of questions
„
How many oxygen atoms in the
following?
– CaCO3
– Al2(SO4)3
„ How many ions in the following?
– CaCl2
– NaOH
– Al2(SO4)3
We count pieces in MOLES.
Types of questions
How many molecules of CO2 are the in
4.56 moles of CO2 ?
22
„ How many moles of water is 5.87 x 10
molecules?
„ How many atoms of carbon are there in
1.23 moles of C6H12O6 ?
24 formula
„ How many moles is 7.78 x 10
units of MgCl2?
„
1
Measuring Moles
Remember relative atomic mass?
„ The amu was one twelfth the mass
of a carbon 12 atom.
„ Since the mole is the number of
atoms in 12 grams of carboncarbon-12,
„ the decimal number on the periodic
table is also the mass of 1 mole of
those atoms in grams.
„
Gram Atomic Mass
The mass of 1 mole of an element in
grams.
„ 12.01 grams of carbon has the same
number of pieces as 1.008 grams of
hydrogen and 55.85 grams of iron.
„ We can right this as
12.01 g C = 1 mole
„ We can count things by weighing
them.
„
Examples
How much would 2.34 moles of
carbon weigh?
„ How many moles of magnesium in
24.31 g of Mg?
„ How many atoms of lithium in 1.00 g
of Li?
22 atoms
„ How much would 3.45 x 10
of U weigh?
„
What about compounds?
„
„
What about compounds?
What is the mass of one mole of CH4?
„ 1 mole of C = 12.01 g
„ 4 mole of H x 1.01 g = 4.04g
„ 1 mole CH4 = 12.01 + 4.04 = 16.05g
„ The Gram Molecular mass of CH4 is
16.05g
„ The mass of one mole of a molecular
compound.
„
in 1 mole of H2O molecules there are two
moles of H atoms and 1 mole of O atoms
To find the mass of one mole of a
compound
– determine the moles of the elements
they have
– Find out how much they would weigh
– add them up
Gram Formula Mass
The mass of one mole of an ionic
compound.
„ Calculated the same way.
„ What is the GFM of Fe2O3?
„ 2 moles of Fe x 55.85 g = 111.70 g
„ 3 moles of O x 16.00 g = 48.00 g
„ The GFM = 111.70 g + 48.00 g = 159.70g
„
2
Molar Mass
The generic term for the mass of one
mole.
„ The same as gram molecular mass,
gram formula mass, and gram atomic
mass.
„
Examples
Calculate the molar mass of the
following and tell me what type it is.
„ Na2S
„ N2O4
„C
„ Ca(NO3)2
„ C6H12O6
„ (NH4)3PO4
„
Molar Mass
The number of grams of 1 mole of
atoms, ions, or molecules.
„ We can make conversion factors
from these.
„ To change grams of a compound to
moles of a compound.
„
Using Molar Mass
Finding moles of compounds
Counting pieces by weighing
For example
„
How many moles
is 5.69 g of NaOH?
NaOH?
⎛
5.69 g⎜⎜
⎝
1 mole
40.00 g
How many moles is 4.56 g of CO2 ?
„ How many grams is 9.87 moles of
H2O?
„ How many molecules in 6.8 g of
CH4?
„ 49 molecules of C6H12O6 weighs how
much?
„
⎞
⎟⎟ = 0.142 mol NaOH
⎠
need to change grams to moles
z for NaOH
z 1mole Na = 22.99g 1 mol O = 16.00 g
1 mole of H = 1.01 g
z 1 mole NaOH = 40.00 g
z
Examples
3
Gases
„ Many
Gases and the Mole
Standard Temperature and
Pressure
„ 0ºC
and 1 atm pressure
„ abbreviated STP
„ At STP 1 mole of gas occupies 22.4 L
„ Called the molar volume
„ Avagadro’
Avagadro’s Hypothesis - at the same
temperature and pressure equal
volumes of gas have the same
number of particles.
of the chemicals we deal with
are gases.
„ They are difficult to weigh.
„ Need to know how many moles of
gas we have.
„ Two things effect the volume of a gas
„ Temperature and pressure
„ Compare at the same temp. and
pressure.
Examples
„ What
is the volume of 4.59 mole of
CO2 gas at STP?
„ How many moles is 5.67 L of O2 at
STP?
„ What is the volume of 8.8g of CH4
gas at STP?
Density of a gas
D = m /V
„ for a gas the units will be g / L
„ We can determine the density of any
gas at STP if we know its formula.
„ To find the density we need the mass
and the volume.
„ If you assume you have 1 mole than
the mass is the molar mass (PT)
„ At STP the volume is 22.4 L.
„
Examples
„ Find
the density of CO2 at STP.
„ Find the density of CH4 at STP.
4
The other way
„ Given
the density, we can find the
molar mass of the gas.
„ Again, pretend you have a mole at
STP, so V = 22.4 L.
„m = D x V
„ m is the mass of 1 mole, since you
have 22.4 L of the stuff.
„ What is the molar mass of a gas with a
density of 1.964 g/L?
„ 2.86 g/L?
All the things we can change
We have learned how to
„ change
moles to grams
to atoms
„ moles to formula units
„ moles to molecules
„ moles to liters
„ molecules to atoms
„ formula units to atoms
„ formula units to ions
„ moles
Volume
Like all percents
„ Part x 100 %
whole
„ Find the mass of each component,
„ divide by the total mass.
PT
Mass
Moles
6.02 x 1023
Representative
Particles
Atoms
Percent Composition
„
22.4 L
Ions
Example
„
Calculate the percent composition of
a compound that is 29.0 g of Ag with
4.30 g of S.
5
Getting it from the formula
If we know the formula, assume you
have 1 mole.
„ Then you know the pieces and the
whole.
„
Examples
Calculate the percent composittion
of C2H4?
„ Aluminum carbonate.
„
The Empirical Formula
„ The
Empirical Formula
From percentage to formula
Calculating Empirical
„ Just
find the lowest whole number ratio
„ C6H12O6
„ CH4N
„ It is not just the ratio of atoms, it is also
the ratio of moles of atoms.
„ In 1 mole of CO2 there is 1 mole of
carbon and 2 moles of oxygen.
„ In one molecule of CO2 there is 1 atom
of C and 2 atoms of O.
lowest whole number ratio of
elements in a compound.
„ The molecular formula the actual
ration of elements in a compound.
„ The two can be the same.
„ CH2 empirical formula
„ C2H4 molecular formula
„ C3H6 molecular formula
„ H2O both
Calculating Empirical
„ Means
we can get ratio from percent
composition.
„ Assume you have a 100 g.
„ The percentages become grams.
„ Can turn grams to moles.
„ Find lowest whole number ratio by
dividing by the smallest.
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Example
„ Calculate
the empirical formula of a
compound composed of 38.67 % C,
16.22 % H, and 45.11 %N.
„ Assume 100 g so
= 3.220 mole
„ 38.67 g C x 1mol C
C
12.01 gC
= 16.09 mole
„ 16.22 g H x 1mol H
H
1.01 gH
„ 45.11 g N x 1mol N = 3.219 mole N
14.01 gN
Example
„ The
ratio is 3.220 mol C = 1 mol C
3.219 molN
1 mol N
„ The ratio is 16.09 mol H = 5 mol H
3.219 molN
1 mol N
„ C1H5N1
„ A compound is 43.64 % P and 56.36
% O. What is the empirical formula?
„ Caffeine is 49.48% C, 5.15% H,
28.87% N and 16.49% O. What is its
empirical formula?
Empirical to molecular
„ Since
the empirical formula is the
lowest ratio the actual molecule
would weigh more.
„ By a whole number multiple.
„ Divide the actual molar mass by the
the mass of one mole of the empirical
formula.
„ Caffeine has a molar mass of 194 g.
what is its molecular mass?
Example
„A
compound is known to be
composed of 71.65 % Cl,
Cl, 24.27% C
and 4.07% H. Its molar mas is known
(from gas density) is known to be
98.96 g. What is its molecular
formula?
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