% Composition and Empirical and Molecular Formulas. Mr. ShieldsRegents Chemistry U04 L04. 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
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Empirical and Molecular Formulas
Mr. ShieldsRegents Chemistry U04 L04
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
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 CoCl2.6H2Ohow many mol of water
molecules would I have if all the water was driven off?
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
Back to Hydrates – moles of Water
How many moles of water are in 3 moles
of CaO∙8H20 ?
3 x 8 = 24 moles
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
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
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%
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 themolecular formula, but they Can be
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
Glucose is a carbohydrate important to sustaining life.
It’s a compound with the molecular formula C6H12O6.
Is it the same as its
What’s the empirical 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.
The chemist has several ways to determine Molar
Mass (Which we won’t go into) without knowing the
Actual 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!
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
78.12/13.02 = 6
The molecular formula is 6x larger than the empirical
Formula. So the molecular formula must be 6x the
Empirical formula or C6H6
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