Formulas and Percent Composition

1
Formulas and Percent Composition

• Finding the Mystery Formulas

2
Using Analytical Data

• Scientist synthesize new compounds for many uses.
  
• Once they make a new product, they must check its
  identity.
• One way to carry out a chemical analysis is by
  providing a percent composition.

3
Example in History

• In 1962, scientists made a new compound from
  xenon and fluorine.
• Before 1962, scientist believed xenon did not
  form compounds.
• The scientists did a percent composition of the
  compound and found it was 63.3 Xe and 36.7 F,
  which is the same as that for a formula XeF4.
• Percent Composition helps identify a substance by
  comparing ratio of masses (empirical formula).

4
Determining Composition

• Composition Mass of part of sample
• Mass of whole sample
• composition remains consistent regardless of
  the size of the sample

X 100
5
Example Comp Problem

• Ex. Find the composition of Ca, O and H in a
  Ca(OH)2 sample.
• Based on a 20.18 g sample of Ca(OH)2, how many
  grams did the individual elements contribute?

6
Determining Empirical Formulas

• Data for a percent composition allow you to
  calculate the simplest ratio among the atoms
  found in a compound.
• Empirical Formula is the simplest ratio of atoms
  in a compound
• Ammonium Nitrite formula is NH4NO2
• Ammonium Nitrite empirical Formula is NH2O

7
Percent Composition To Empirical Formula

• Step 1 change to grams, assuming a 100 g
  sample
• Step 2 convert grams to moles
• Step 3 divide all mole values by the smallest
  mole value to try and get all values into simple
  whole numbers
• Step 4 if mole values are not whole numbers
  after step 3, multiply all mole values by the
  same smallest whole number to achieve this 
• Step 5 write the empirical formula of the
  compound
•  

8
Example Problems

• A compound is found to have 79.8 carbon and
  20.2 hydrogen by mass. What is the empirical
  formula of this compound?

9

• A compound is composed of 58.8 C, and 9.8 H.
  What is the empirical formula of this compound?

10
More Practice Problems

• 60.0 C, 13.4 H, 26.6 O. Find the empirical
  formula.
• A dead battery is found to contain a compound of
  69.9 Mn and 30.4 O. What is the empirical
  formula?
• Magnetic iron oxide is 72.4 iron and 27.6
  oxygen. What is the empirical formula?

11
Empirical formulas not molecular formulas

• In determining empirical formulas, the simplest
  ratio of atoms was determined. This is not
  necessarily the exact number of atoms in the
  compound which is known as the molecular formula.
  
• Additional information is needed to determine
  molecular formulas.

12
Molecular Formulas

• Molecular Formulas are made of single molecules.
• For many compound the molecular formula is a
  simple whole number ratio of the empirical
  formula
• The molar mass of a molecular formula is equal to
  the molar mass of a empirical formula multiplied
  by n (a whole number)

13
Process

• If you divide the experimental molar mass by the
  molar mass of the empirical formula, you can
  figure out the value of n needed to scale the
  empirical formula up to give the molecular
  formula.
• n(emperical formula) molecular formula

14
Example Problem

• The empirical formula of a compound is CH3N. The
  mass of the molecular formula is 60.12 g/mol.
  What is the molecular formula of this compound?

15
Steps To Find The Molecular Formula From The
Empirical Formula

1. Find molar mass of the empirical formula
2. Divide the molecular mass given in the problem by
   the empirical formula mass to get the multiple of
   how many times larger the molecular mass is
   compared to the empirical mass (n).
3. Use the multiple you found in step 2 to write
   your molecular formula. This must mean that the
   of atoms of the empirical formula must be
   multiplied by this multiple n(empirical formula)

16
More Example Problems

• Molar mass of 78 g/mol and empirical formula of
  CH, what is its molecular formula?
• A brown gas on the cabinet to your right has the
  empirical formula of NO2. Its experimental molar
  mass is 46g/mol. What is its molecular formula?

17

• The lichens presents on the bark of trees contain
  a substance called succinic acid. The organic
  acid is also present in fungi, which is used to
  make dyes and pefumes.
• Percent composition is 40.68 C, 5.08 H, 54.24
  O.
• Its molecular mass is 118.8 g/mol
• What is its empirical and molecular formulas?