Transcript Chemical Composition Chapter 8

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Dalton used the percentages of
elements in compounds and the
chemical formulas to deduce the relative
masses of atoms
Unit is the amu(atomic mass unit)
◦ 1 amu = 1.66 x 10-24g
We define the masses of atoms in terms
of atomic mass units
◦ 1 Carbon atom = 12.0 amu,
◦ 1 Oxygen atom = 16.0 amu
◦ 1 O2 molecule = 2(16.00 amu) = 32.00 amu
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Atomic masses allow us to convert weights into
numbers of atoms
If a sample of carbon weighs 3.00 x 1020 amu how
many atoms of carbon are present?
Calculate the Mass (in amu) of 75 atoms of Al
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The mass of 1 Al atom = 27.0 amu
Use the relationship as a conversion factor
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A mole is a counting unit, just like a dozen,
except that a mole is much larger.
A mole (mol) is a number that represents the
number of representative particles of a
substance.
It is always equal to 6.02 x 1023 and is
referred to as Avogadro’s number.
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Atom - single element, for example: Na, C
Diatomic molecule – H2, N2, O2, F2, Cl2, Br2, I2
Molecule – contains no metals, for example:
H2O, C12H22O11
Ion – only ions, for example, Ca2+, P3Formula Unit – always starts with a metal, for
example: CaF2, CuSO4
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Avogadro’s # can be used as a conversion
factor where 1 mol= 6.02 x 1023 atoms, ions,
molecules, or formula units depending on the
type of substance you have.
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Convert 4.27 x 1023 molecules of CO2 to
moles
How many formula units are there in 2.78
moles of CaCl2?
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How many molecules are there in 0.85 moles
of octane, C8H18?
How many moles are there in 7.6 x 1022 Na+
ions?
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When converting between grams and moles,
1 mole of an element equals the average
atomic mass of that element in grams,
rounded to one number after the decimal.
◦ 1 mol Ca = 40.1 g Ca
1 mol P = 31.0 g P
◦ 1 mol Cl = 35.5 g Cl
1 mol K = 39.1 g K
◦ 1 mol S = 32.1 g S
1 mol Cu = 63.6 g Cu
◦ 1 mol Br = 79.9 g Br
1 mol Si = 28.1 g Si
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How many moles are there in a 10.0 gram
sample of Al?
How many grams are there in a 0.643 mol
sample of Fe?
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How many formula units of Al2O3 are there in
0.750 moles of Al2O3? How many atoms of
each element are in the sample of Al2O3?
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Molar mass – mass of 1 mol of a pure
substance usually written in grams per mole
(g/mol)
The subscript next to each element in a
formula tells you how many moles of that
particular element are in the compound.
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When finding molar mass of a compound if
there are parentheses in the formula, you
must multiply each element in the
parentheses times the number outside to
determine the total number of moles of each
element.
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Determine the molar mass for:
Fe2(SO4)3
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Cu(ClO3)2
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Ni(C4H4O6)2
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Determine the number of moles in a
22.3 gram sample of Al2(CO3)3
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How many grams of ammonium phosphate,
(NH4)3PO4, are present in a 0.082 mole
sample of (NH4)3PO4?
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How many grams of gold, Au, are present in a
8.21 x 1022 atom sample of Au?
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Determine the number of formula units are
there in a 47.9 gram sample of Mg(NO3)2
To determine percent composition
1. Determine molar mass of your compound
(g/mol).
2.
Divide the total mass for each element by
the molar mass of the compound.
3.
Multiply each element’s % by 100.
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If you add up the percentages for all of
the
elements, you should get very close to
100%.
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Determine the percent composition for each
element in the following compounds:
◦ K2SO4
◦ Ba3(PO4)2
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Empirical formula – lowest whole-number
ratio of the moles in a compound.
Molecular formulas are simple multiples of
empirical formulas that can be reduced if
each element in the compound is divisible by
the same whole number.
Empirical formulas cannot be reduced any
further and still have whole numbers for each
element in the compound.
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C6H9O3
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C12H22O11
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C4H6O2
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C3H8N4
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Ca3(S2O3)2
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3.
If given percentages for each element,
convert each one directly to grams.
Divide the grams of each element by its
molar mass to get moles of each element.
Divide each number of moles by the
smallest number of moles.
4.
All answers must be within 0.1 of a whole
number. If not, then all of the elements
must be multiplied by a small whole number
so that all elements are whole numbers.
If 0.5, then multiply all elements by 2
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A compound is composed of 50.0% S and
50.0% O. What is the empirical formula of
this compound?
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A compound contains 33.88% Cu, 14.94% N,
and the remaining % is Oxygen. What is the
empirical formula of this compound?
Determine the Empirical Formula of a compound
that contains 55.2% Carbon, 2.3% Hydrogen ,
18.4% Oxygen and 24.1% Nitrogen
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Determine the empirical formula for the
compound.
Determine the molar mass of the empirical
formula.
Divide the molar mass of the molecular
formula over the molar mass of the
empirical formula.
Multiply the answer from #3 by the
empirical formula.
Molar Mass of Molecular Formula
---------------------------- = X
Molar Mass of Empirical Formula
Molecular Formula = (Empirical formula)X
 To check to see if your molecular formula is
correct, determine the molar mass of your
molecular formula. It should be equal to the
molar mass of the molecular formula you
were given in the problem.
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Example: A compound has an empirical
formula of C2H3O. What is the molecular
formula if the molecular formula mass is
172.0 g/mol?
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Determine the empirical and molecular
formulas for a compound containing 94.1%O
and 5.9% H. The molar mass of the molecular
formula equals 34.0 g/mol.
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Determine the empirical and molecular
formulas for a compound containing 26.1%C,
50.7% N and 23.2% O. The molar mass of the
molecular formula equals 828.0 g/mol.