Empirical and Molecular Formulas

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Empirical and Molecular Formulas
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Chemistry Joke
Q What happens to rock that has been heated to
6.02 X 1023 degrees?
A It becomes molten!
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Definitions

• Empirical formula the lowest whole-number ratio
  of the atoms of the elements in a compound.
• Molecular formula chemical formula that shows
  the actual number of atoms in a compound.

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Which is it empirical, molecular, or possibly
both??
NO
N2O4
C6H12O6
Ba(OH)2
C4H8
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Empirical Formulas

• The simplest whole-number ratio of atoms in a
  compound.

H2O2
HO
Both are divisible by 2.
H2O
H2O
Already simplified
CH2O
C6H12O6
All are divisible by 6.
CaCl2
Correct ionic formulas are always empirical.
CaCl2
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Finding an Empirical Formula

1. Grams ? mole
2. Divide by smallest
3. Round til whole

An unknown compound contains 0.0806 grams of
C 0.01353 grams of H 0.1074 grams of O What is
the empirical formula of the compound?
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Step 1 Change grams to moles
0.0806 g C
0.01353 g H
0.1074 g O
1 mol O
0.0806 g C
0.01353 g H
1 mol H
0.1074 g O
1 mol C
1.01 g H
16.00g O
12.01 g C
0.0068 mol C
0.01340 mol H
0.0067 mol O
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Step 2 Divide by smallest mole value Step 3
Round to the whole
0.0068 mol C
0.01340 mol H
0.0067 mol O
0.0067
0.0067
0.0067
1.0149 mol C
2 mol H
1 mol O
1 mol C
2 mol H
1 mol O
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1 mol C
2 mol H
1 mol O
CH2O
This is the empirical formula.
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• Lets try another
• This time, using
• percent composition!!

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An unknown compound is known to have the
following composition 11.11 hydrogen 88.89
oxygen What is the empirical formula of the
compound?
In these problems we will choose to use a 100
gram sample. So, in this sample we have 11.11
grams H and 88.89 grams O.
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Step 1 Change grams to moles.
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Step 2 Divide by smallest mole value. Step 3
Round to a whole number.
5.56 mol O
11.00 mol H
5.56
5.56
2
1
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The ratio of moles of hydrogen to moles of oxygen
is 21
H2O
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Going one step furtherFinding molecular
formulas from empirical formulas.
Remember The empirical formula is the simplest
whole number ratio whereas, the molecular
formula shows the actual number of each atom.
Step 1 Find molar mass of empirical formula.
Step 2 Divide the molar mass of the compound by
the molar mass of the empirical formula.
Step 3 Multiply the empirical formula by that
number.
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Practice Problem

• CH2O is the empirical formula. The molar mass of
  the actual compound is 180 g/mol. What is the
  molecular formula?

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Step 1 Find molar mass of empirical formula.
(1 x 12.01) (2 x 1.01) (1 x 16.00) 30. 03
g/mol
Step 2 Divide the molar mass of the compound by
the molar mass of the empirical formula.
180 g / 30 g 6
Step 3 Multiply the empirical formula by that
number.
6 (CH2O)
C6H12O6 The Molecular Formula
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You try one

• Empirical formula CH4
• The actual molecular mass is 64g.

Find the molar mass of empirical formula ? C
4(H) 16g
Divide molecular mass by empirical mass ? 64g /
16g 4
Multiply the empirical formula by that number ? 4
(CH4)
C4H16 ? molecular formula
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One more thing to mention

• Hydrates vs. Anhydrates
• Hydrates salts with water molecules attached
• BaCl2 2 H2O
• Name the salt, then use a prefix word
  hydrateBarium Chloride Dihydrate
• Anhydrates the salt without its water
• BaCl2

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Finding the Formulas of Hydrates--Similar to
Finding an Empirical Formula

• 1. Find the mass of the water by subtraction.
• The mass of the hydrate and the anhydrate will be
  given.
• 2. Change g to mol
• Mass of water to mol
• Mass of salt to mol
• 3. Divide by the smallest mole value
• 4. Round to a whole if necessary.

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Finding the Formulas of Hydrates
BaCl2 X H2O
Hydrate mass (mass with water) 24.40
g Anhydrate mass (mass w/o water) - 20.80 g
Step 1 Find the mass of the water.
Mass H2O 24.40 g 20.80 g 3.60 g H2O
For the formula we want the ratio of moles of
water to moles of anhydrate.
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Step 2 Change grams to moles
Grams of Salt
Grams of Water
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Step 3 Divide by the smallest mole value.
0.1 mol BaCl2
0.2 mol H2O
0.1
0.1
1
2
Step 4 Round to a whole if necessary.
BaCl2 2 H2O
Barium chloride dihydrate
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Calculating Percent Water in a Hydrate
Calculating the percent water in a hydrate is
similar to calculating percent composition
H2O mass of the water x 100
mass of the hydrate
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Example Problem
In the lab, a five gram sample of hydrous copper
(II) nitrate is heated. If 3.9 grams of the
anhydrous salt remains, what is the percent water
in the hydrate?
Mass of water 5 g 3.9 g 1.1 g H2O
Mass of hydrate 5 g
22 H2O
(1.1 g H2O / 5 g total) x 100
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Chemistry Joke
Q What is a dog lovers favorite part of
chemistry?
A The Lab Work!