1. Ch. 7: Chemical Formulas and Compounds
7.3 Uses of Chemical Formulas

2. Meaning of Formulas the subscripts in a formula give you:
simplest ratio of atoms OR
the number of atoms in a molecule
ALWAYS provides you with a ratio of moles of atoms
Example: C13H18O2
13 moles of C : 18 moles of H : 2 moles of O
ratio can be used as a conversion factor

3. Example
If I have 2.00 moles of C13H18O2, how many moles of each atom would I have?

4. Formula Mass mass of a molecule, ion, or formula unit
sum of mass of all atoms in the chemical formula
in amu
Ex: H2O
amu
formula mass = molecular mass for molecular compound

5. Example Find formula mass of potassium chlorate. KClO3
1( ) + 1( ) + 3( )
amu

6. Molar Masses mass of one mole of pure substance
numerically equal to formula mass
units: g/mol
Find molar mass of barium nitrate.
Ba(NO3)2
1( ) + 2( ) + 6( )
g/mol

7. Molar Mass in Conversions
can be used as a conversion factor
between grams and moles
What is the mass in grams of 2.50 mol of oxygen gas?

8. Example
Ibuprofen, C13H18O2, is the active ingredient in many pain relievers.
Find molar mass:
(13 x ) + (18 x ) + (2 x )
= g/mol

9. Example
If the tablets in a bottle contain a total of 33 g of ibuprofen, how many moles are in one bottle?
How many molecules of ibuprofen are in the bottle?

10. Example What is the number of moles of carbon in that bottle?
What is the total mass in grams of carbon in the bottle?

11. Conversion with Compounds
grams and moles
molar mass of compound
moles and number of molecules or units
Avogadro’s number
moles and moles
mole ratio from subscripts

12. Percentage Composition
percentage by mass of each element in the compound
assume you have 1 mole of sample total
convert moles to grams for each element
divide grams of each by molar mass of compound
multiply by 100

13. Example Find percent composition for C13H18O2
Assuming 1 mole of sample, find grams of each element

14. Example
Divide each mass by total molar mass and multiply by 100