Title: DETERMINING CHEMICAL FORMULAS
1DETERMINING CHEMICAL FORMULAS
2What if you discovered the cure for cancer?
- Any new chemical that is discovered that is a
potential health care treatment must have its
molecular formula submitted to the Ministry of
Health. - How do you determine the molecular formula of
some new compound that neither your nor anyone
may have never seen before? - Its actually not that hardall you need is a
device called a mass spectrometer and some
chemistry fundamentals some of which are
hundreds of years old!
3The Mass Spectrometer
- A mass spectrometer is a lab instrument that
measures the molar mass of a compound. - The machine can also give you a reading of each
element in the compound based on mass and
percentage. - Look at this reading from the Cassini Spacecraft
as it flew by one of Saturns moons Enceladus.
4Law of Constant Composition
- A compound contains elements in certain fixed
proportions (ratios) and in no other
combinations. - This law is important in formula determination
because it basically says that each compound has
only one formulaif you find something that
matches a known formula then that is what you
have found no arguments. - Water will always be H2OFind that and you have
found water.
5Empirical vs. Molecular
- Before you can determine the molecular formula of
a compound you must first determine the empirical
formula. - An empirical formula is the lowest whole number
ratio of elements in a compound. - Think of empirical as a formula in lowest terms.
- The molecular formula of a compound is the actual
number of atoms in one molecule of a compound. - Think of molecular formulas as seeing the
compound out on the street. - Example Glucose C6H12O6 (molecular formula)
- CH2O (empirical formula)
6Determining Empirical Formulas
- In order to determine the empirical formula for a
compound you need the percent composition for it - The mass spectrometer will give you thisor the
question will tell you! - Lets try an example of a problem in which we
must determine the percent composition.
7Sample Problem 1 The substance that gives sour
milk that lovely taste is lactic acid. It
consists of 40.00 Carbon and 6.71 Hydrogen with
the remainder being oxygen. Determine its
empirical formulaOr Else!!!
- Steps to Solve
- Assume 100g
- ( ? g)
- g ? mol
- ( by g/mol)
- Divide by lowest mol
- C 40.00g 12.01g/mol 3.33mol
- C 3.33mol 3.33mol 1
- H 6.71g 1.01g/mol 6.64mol
- H 6.64mol 3.33mol 2
- O 53.29g 16.00g/mol 3.33mol
- O 3.33mol 3.33mol 1
- Therefore the empirical formula for lactic acid
is CH2O.
8Empirical Is Not Enough!
- If you look at this slideshow carefully you
will have noticed both glucose (C6H12O6) and
lactic acid (C3H6O3) have the same empirical
formula but they are very different molecules
it terms of properties and function within the
human body! - Many molecules can have the same empirical
formulas so it is absolutely necessary to be able
to determine the molecular formula for any
compound. - In order to determine the molecular formula we
need two pieces of information - Empirical Formula (or a way to get itlike
percent compositiona la the mass spectrometer) - Molar mass of the compound (The almighty mass
spectrometer will give us this too!)
9Behold!...The Mass Spec!
10Sample Problem 2 Determine the molecular formula
of a compound containing 85.7 carbon and 14.3
hydrogen by mass. The molar mass of the compound
is 84g/mol. SolveOr Else!!!
- We do not have the empirical formula but we do
have a way to get it. (by using the Percent
Composition)! - C 85.7g 12.01g/mol 7.136mol 7.136mol 1
- H 14.3g 1.01g/mol 14.16mol 7.136mol 2
- Therefore the empirical formula is CH2.
- Molar Mass 84g/mol 6
- Empirical Formula Mass 14.03g/mol
- This means that the molecular formula is 6 times
larger than the empirical formula so we need to
multiply the empirical formula by 6 to get the
molecular formula. - 6 x CH2 C6H12
- The Molecular formula is C6H12.
11Sample Problem 3 Determine the molecular formula
of a compound with the empirical formula P2O3 and
a molar mass of 220.0g/mol.
- We are given the empirical formula outright so
you go directly to the second step of the solving
process. - Molar Mass 220.0g/mol 2
- Empirical Formula Mass 109.94g/mol
- This means that the molecular formula is 2 times
larger than the empirical formula so we need to
multiply the empirical formula by 2 to get the
molecular formula. - 2 x P2O3 P4O6
- The molecular formula is P4O6.
12Thats All I Got!...
- I know its a lot to think about
- So I will leave you to it.