Mr. Shields
Regents Chemistry
U04 L04
1
Molar Composition
When we see a chemical formula we immediately know how
Many and what kind of atoms are in the compound.
But number of atoms can also represent # of moles of atoms
In the compound. For example…
In one mole of NaHCO3 I have 1 mol of Na, 1 mol of H, 1 mol
of C and 3 moles of oxygen atoms
And in 0.25 mol of NaHCO3 I can calculate that I have
0.25 mol of Na, 0.25 mol of H, 0.25 mol of C and 0.75 mol of
2
Oxygen atoms
Back to Hydrates
Now try this problem…
How many moles of water are there in 0.67 mol of CoCl2.6H2O
Think of it this way…
If I had 1 mol of CoCl2.6H2O how many mol of water
molecules would I have if all the water was driven off?
CoCl2.6H2O
CoCl2
HEAT
+
H20
H20
H20
H20
H20
H20
Now, If I only had 0.67 mole of CoCl2.6H2O how
many mol of water would I have? 0.67 x 6 = 4.02 mol
3
Back to Hydrates – moles of Water
Problem:
How many moles of water are in 3 moles
of CaO∙8H20 ?
3 x 8 = 24 moles
Problem:
Two moles of water were driven off when a sample of
CaO∙8H20 was heated leaving the anhydrous salt behind.
How many moles of hydrate were originally present?
IF I had 1 mole of hydrate I would have 8 moles of water but
I only got 2 moles of water so I only had 2/8 = 0.25 moles 4
Moles of atoms in hydrates
When a compound has NO water of hydration it is
said to be Anhydrous and is called an Anhydride.
Here’s another problem involving Hydrates…
A 25 gram sample of a hydrate is heated until all the water is
driven off. The mass of the anhydrous product remaining was
14 grams. What is the % water in the hydrate?
Ans: 25 g of hydrate – 14 grams of remaining anhydrous
Product = 11 grams of water
11g H20 / 25g Hydrate = 44% water
5
Hydrates
Problem: if a 15g sample of a hydrate is heated
until all the water of hydration is driven off. The
mass of the anhydrous product remaining is 9.5
grams. What is the percent of water in the
hydrate?
Answer:
15g - 9.5g = mass of the water = 5.5g
% water = (5.5g/15g) x 100 = 36.67%
6
Empirical formula
DEFINITION:
A chemical formula that states the simplest whole
number ratios of atoms in a compound is
called the EMPIRICAL FORMULA of the compound
Ex. C8H16  CH2
Empirical formula’s ARE NOT NECESSARILY the same
as the molecular formula, but they Can be
7
Empirical formula
Ex. C6H6 (Benzene) is NOT an empirical formula
it’s the true Molecular Formula. Why?
BUT Consider Methane (CH4)
It’s molecular formula is also it’s empirical formula. Why?
What are the empirical formula’s of the following? Is the
Empirical formula the same as the molecular formula
C2H4Cl2
C3H3Br3
C2H6O
8
Glucose is a carbohydrate important to sustaining life.
It’s a compound with the molecular formula C6H12O6.
What’s the
Empirical formula?
Is it the same as its
Molecular formula?
glucose
9
Empirical formulas
The molecular formula for Hydrogen Peroxide
is H2O2. What’s the empirical formula?
The simplest whole number ratio of the elements
is HO. This is it’s empirical formula
What’s the empirical formula
for Water?
H 2O
10
Molecular Formula
Obviously a chemist needs to know whether an empirical
Formula is or is not also the molecular formula.
For example is water H20 or H4O2 or H6O3
To determine this the Chemist needs one more piece
of information. They need the compound’s molar mass.
NOTE:
The chemist has several ways to determine Molar
Mass (Which we won’t go into) without knowing the
Actual molecular formula
11
Molecular Formula
Let’s look at Benzene as an example
Benzene has an empirical formula of CH therefore the
Molar mass is 13.02
?!
If the chemist determines that the true molar
mass is 78.12 can the empirical formula also be
the molecular formula?
NO, the molar masses do not match!
12
Molecular Formula
So what is the true molecular formula?
If the true molar mass of Benzene is 78.12 and the
Molar mass of the empirical formula is 13.02 how
much greater is the molecular mass than the empirical
Mass?
78.12/13.02 = 6
The molecular formula is 6x larger than the empirical
Formula. So the molecular formula must be 6x the
13
Empirical formula or C6H6
Molecular Formulas
Let’s try another problem.
A compound has an empirical formula
of CH2O. It’s Molar mass is determined
to be 180g/mol.
What is the molecular formula?
Step 1: Empirical molar mass = 30g
Step 2: Divide molar mass of the empirical formula into
the actual molar mass. 180/30 = 6
Step 3: Multiply all atoms in the empirical formula by 6
This is the molecular formula
14
C6H12O6