Hydrates
Some salts (ionic compounds) trap
water when they form crystals
– these are hydrates.
Both the name and the formula needs to
indicate how many water molecules are
trapped
In the name we add the word hydrate
with a prefix that tells us how many
water molecules
Hydrates
In the name we add the word hydrate
with a prefix that tells us how many
water molecules
– Mono, di, tri, tetra, penta, hexa, hepta
– Calcium chloride dihydrate =
CaCl2•2Η 2Ο
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Hydrates
In the formula you put a dot and then
write the number of molecules.
– Chromium (III) nitrate hexahydrate =
Cr(NO3)3• 6H2O
Atomic Mass
Atoms are so small, it is difficult to
discuss how much they weigh in grams
– Use atomic mass units.
an atomic mass unit (amu) is one
twelfth the mass of a carbon-12 atom
– 1 amu = 1.66x10-24 g
This gives us a basis for comparison
The decimal numbers on the table are
atomic masses in amu
They are not whole numbers
Because they are based on averages of
atoms and of isotopes.
can figure out the average atomic mass
from the mass of the isotopes and their
relative abundance.
add up the percent as decimals times
the masses of the isotopes.
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Examples
There are two isotopes of carbon 12C with
a mass of 12.00000 amu(98.892%), and
13C with a mass of 13.00335 amu (1.108%)
There are two isotopes of nitrogen , one
with an atomic mass of 14.0031 amu and
one with a mass of 15.0001 amu. What is
the percent abundance of each?
The Mole
The mole is a number
a very large number, but still, just a
number
6.02 x 1023 of anything is a mole
a large dozen
The number of atoms in exactly 12
grams of carbon-12
For
Small
Amounts of Particles
Use the inverse of Avogadro’s number
Instead of 1 mol = 6.02 X 1023 particles
Use 1 amu = 1.66x10-24 g
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Example
What is the mass in grams of five
molecules of Phophorus Pentafluoride?
Answer: 1.05 x 10 -21 g
The Mole
Makes the numbers on the table the
mass of the average atom
Average atomic mass
Just atomic mass
Molar mass
mass of 1 mole of a substance
Mass/Mol
– Ionic Compounds:
» Formula Weight
– Molecular Compounds:
» Molecular Weight
– Elements:
» Average Atomic Mass
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Molar Mass
To determine the molar mass of an
element, look on the table.
To determine the molar mass of a
compound, add up the molar masses of
the elements that make it up.
Find the molar mass of
CH4
Mg3P2
Ca(NO3)2
Al2(Cr2O7)3
CaSO4 · 2H2O
Molar Mass and Avogadro’s
Number as a Conversion Factor
The molecular formula of allicin, the
compound responsible for the
characteristic smell of garlic, is
C6H10OS2. How many molecules of
allicin are in 5.00 mg of the substance?
How many S atoms are present in 5.00
mg of allicin?
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Percent Composition
The percent composition is the mass
percentage of each type of atom(element) in a
compound.
% X = (total atomic mass of X / molar mass which contains X)
Percent Composition Example
Calculate the percent composition of nicotine,
C10H14N2.
•
•
•
•
Molar mass = 10C + 14H + 2N = 162.26 g/mol
%C = (10C / C10H14N2)100
• = (120.10 g/mol /162.26g/mol)100=74.0%
%H = (14H / C10H14N2)100
• = (14.14 g/mol / 162.26 g/mol)100 = 8.7%
%N = (2 N / C10H14N2)100
• = (28.02 g/mol / 162.26 g/mol)100 = 17.3%
Percent Composition
Examples
Find the percent composition of
– CH4
– Al2(Cr2O7)3
– CaSO4 · 2H2O
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Percent Composition Example
•
•
•
A student prepares a compound of tungsten chloride
from 3.946 g of tungsten and 3.806 g of chlorine.
Assuming the reaction goes to completion, calculate the
percent composition.
According to the Law of Conservation of Mass, 3.946 g
of W combines with 3.806 g of Cl to give 7.752 g of
compound.
%W = (mass W / mass WxCly )100
• = (3.946 g / 7.752 g)100 = 50.9%
%Cl = (Cl / WxCly )100
• = (3.806g / 7.752 g)100 = 49.1%
Percent Composition Example
•
•
•
•
•
•
•
How many grams of lithium will combine with 20.0 g of
sulfur to form Li2S?
Molar mass of Li2S
• = 2Li + S = 2(6.94 g/mol) + 32.06 g/mol = 45.94 g/mol
% composition of Li in Li2S
• = (mass of Li/molar mass)100
• =(13.88 g/mol / 45.94 g/mol) 100 = 30.2%
% composition of S in Li2S
• = 100% - 30.2% =69.8%
A ratio can now be established:
20.0 g S = 69.8%
x = (30.2%) (20.0 g) x = 8.65 g Li
x g Li
30.2%
69.8%
Working backwards
From percent composition, you can
determine the empirical formula.
Empirical Formula the lowest ratio of
atoms in a molecule
Based on mole ratios
A sample is 59.53% C, 5.38%H,
10.68%N, and 24.40%O what is its
empirical formula.
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Pure O2 in
Sample is burned
completely to
form CO2 and
H2O
CO2 is absorbed
H2O is absorbed
Example Problem
A 0.2000 gram sample of a compound
(vitamin C) composed of only C, H, and
O is burned completely with excess O2 .
0.2998 g of CO2 and 0.0819 g of H2O are
produced. What is the empirical
formula?
Empirical To Molecular
Formulas
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Empirical To Molecular
Formulas
Examples:
Empirical Formula
Molar Mass
Molecular Formula
Multiplier
Example
A compound is made of only sulfur and
nitrogen. It is 69.6% S by mass. Its molar
mass is 184 g/mol. What is its formula?
Example
1.92g M+ ion reacts with 0.158 mol Xion to produce a compound, MX2,
which is 86.8% X by mass. What are the
identities of M+ and X-?
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