Empirical and Molecular Formulas

1
Empirical and Molecular Formulas

• How to find out what an unknown compound is

2
A chemist obtains a new product

• What is the formula for the compound?
• First step determine constituent elements and
  their amounts
• This info can be used to determine chemical
  formula

3
Formula of a compound represents relative numbers
of atoms present

• E.g. CO2 tells us that in a molecule of this
  compound there is 1 carbon atom to every 2 oxygen
  atoms
• To determine the formula of a substance we need
  to count the atoms we can do this by weighing

4
An example

• You have a compound that you know contains only
  Carbon, Hydrogen and Oxygen.
• You have a 0.2015 g sample
• Analysis shows you have 0.0806 g C, 0.01353 g H,
  and 0.1074 g O
• We can convert these masses to moles, and then
  moles to atoms using dimensional analysis..

5
Determining of moles of elements in unknown
compound
6
Determining number of atoms of unknown compound
7
To summarize up to this point

• We have .00671 moles of Carbon
• We have .01342 moles of Hydrogen
• We have .006713 moles of Oxygen
• Amount of Carbon Amount of Oxygen
• .01342/.006713 2, so we have twice as much
  Hydrogen as we have of Oxygen and as we have of
  Carbon
• We have a ratio of 121

8
We know the ratio of elements

• 1 Carbon 2 Hydrogens 1 Oxygen
• We can write this as CH2O
• Is this the molecular formula? Maybebut the
  molecule might also have 2 Carbons, 4 Hydrogens
  and 2 Oxygens, or it might have 16 Carbons, 32
  Hydrogens and 16 Oxygens
• We have found the EMPIRICAL FORMULA a formula
  that represents the ratio of elements in a
  compound.
• This is also called simplest formula since it
  is smallest whole-number ratio of elements in the
  compound

9
Empirical Formula Vs Molecular Formula

• Empirical formula gives relative numbers of atoms
  e.g. CH2O
• Molecular formula gives the actual numbers of
  atoms e.g. C6H12O6
• C6H12O6 (CH2O)6

10
Find the Empirical Formula

• Empirical Formula of Benzene CH
• Empirical Formula of Dioxin C6H2Cl2O

11
How to calculate empirical formula

• We have 0.2636 grams of nickel. We heat it in
  the presence of oxygen to produce 0.3354 grams of
  a nickel oxide. What is the formula of the
  compound we made?
• First - what is the mass of oxygen that reacted
  with the copper?
• Mass of copper oxide Mass of copper mass of
  oxygen
• 0.3354 g copper oxide 0.2636 g copper 0.0718
  g oxygen
• Next Find number atoms involved

12
Find number of atoms in the compound

• Mole quantities represent number of atoms
• We have same number of moles of oxygen as of
  nickel
• Empirical formula will be NiO

13
Another example

• You have a metal oxide made by reacting 4.151g Al
  with 3.692 g O. What is the empirical formula?
• Atomic mass Al 26.98 g/mol
• Atomic mass O 16.00 g/mol
• Need to know relative numbers of atoms, so need
  to convert grams to moles and then find the whole
  number ratio of atoms

14
Find whole number ratios

• Do this by dividing both numbers by the smallest
  of the two. This converts the smallest number to
  1

This is not a whole number ratio to get to a
whole number, all we have to do is multiply by 2
15
Finally we have found our empirical formula

• Al2O3

16
Summary
17
If you have percent composition your can also
find the empirical formula

• You have a compound that is 27 Carbon and 73
  Oxygen by mass. What is the empirical formula?
• Assume you have 100 grams. That means you would
  have 27 g C and 73 g O
• Convert grams to moles
• 27 g C (1 mol/12 g C) 2.25 mol C
• 73 g O (1 mol/16 g O) 4.6 mol O
• Divide by smallest of moles
• 4.6 /2.25 2.04 (O)
• 2.25/2.25 1 (C)
• Empirical formula CO2

18
A poem to help

• Percent to mass
• Mass to mole
• Divide by small
• Multiply til whole

19
Find empirical formula from percent composition

• Nylon-6 is a compound that is 63.68 C, 12.38 N,
  9.80 H and 14.4 O. Find the empirical formula

C6H11NO
20
One more step to find molecular formula

• To find the molecular formula of your unknown
  compound, you need another piece of information
  the molar mass of the compound, in addition to
  the percent composition

21
You have the empirical formula what is the
molecular formula?

• Empirical formula P2O5
• Molar mass of compound (determined
  experimentally) is 238.88 g/mol
• P 30.97 g/mol O 16.00 g/mol
• Find empirical formula mass - the molar mass
  that the compound would have based on the
  empirical formula
• 2(30.97g/mol) 5(16.00 g/mol)
• 141.94 g/mol

22
Finding molecular formula

• The molar mass of the compound will be some
  multiple of the empirical mass find the
  multiple by dividing molar mass by empirical
  mass
• (238.88 g/mol)/(141.94 g/mol) 1.7
• 1.7 2
• Molar mass of unknown is about twice empirical
  mass. We will multiply empirical formula
  subscripts by 2
• Molecular formula P4O10

23
One last problem
24
Homework

• Read Ch 6, section 3 (pp. 196 208)
• Do empirical formula homework sheets
• Do molecular formula homework sheets
• Monday we will go over homework and talk about
  hydrates
• Test Review
• Test on Wednesday!!!!
• We have 12 class meetings to go until semester
  final