1. Mathematics of Chemical Formulas
Objectives:
determine the atomic mass of elements and the formula mass of compounds
determine the gram atomic mass of elements and the gram formula mass of compounds
Calculate the mass of a given number of moles of a substance
Calculate the number of moles in a given mass of a substance
Calculate the number of molecules in a given number of moles of a substance
Calculate the volume of a given mass of a gaseous substance at STP
Determine the percentage composition of a substance from its formula
Determine the empirical formulas and molecular formulas of compounds.
Created by C. Ippolito
Dec 2006

2. Stoichiometry
study of the quantitative relationships that can be derived from chemical formulas and chemical equations
chemical formulas
represent substances
chemical equations
represent reactions between substances
Created by C. Ippolito
Dec 2006

3. Formula Mass Formula Mass (molecular mass)
sum of the atomic masses of all of the atoms represented by the formula
Find the Formula Mass of Ca(OH)2
Element Mass Atoms Total
Ca 40 amu x 1 = 40 amu
O 16 amu x 2 = 32 amu
H amu x 2 = 2 amu
74 amu
Created by C. Ippolito
Dec 2006

4. Gram Atomic Mass Gram atomic mass Gram formula mass
that quantity of an element that has a mass in grams equal to its atomic mass
Gram formula mass
that quantity of a substance that has a mass in grams equal to its formula mass
Gram molecular mass
equivalent to formula mass for molecular substances
Created by C. Ippolito
Dec 2006

5. Mole 6.02 x 1023 atoms (a.k.a Avogadro’s number)
# of atoms in a gram atomic mass of any element
# of formula units in a gram formula mass of any substance
gaseous substances at STP [0oC and 1 atm (101.3 kPa)]
has a volume 22.4 liters
12/21 section 1 & 2
Created by C. Ippolito
Dec 2006

6. Percentage Composition
the percentage by mass of each element in a compound
determined by experimentation
Created by C. Ippolito
Dec 2006

7. Formula Determination
Formula determined if one knows:
mass of each element in a sample or
the percentage composition
Example: In a 9.2-g sample 2.8 g is nitrogen and 6.4 g is oxygen. What is the empirical formula?
Created by C. Ippolito
Dec 2006

8. Formula Determination from Masses
Example: In a 9.2-g sample 2.8 g is nitrogen and 6.4 g is oxygen. What is the empirical formula?
Find mole ratio:
nitrogen  oxygen
Created by C. Ippolito
Dec 2006

9. Formula Determination from Percentage Composition
Example: What is the empirical formula of a compound with a percentage mass of 65.2% arsenic and 34.8% oxygen by mass?
Find mole ratio:
arsenic  oxygen
sect 3 jan 2
Created by C. Ippolito
Dec 2006

10. Empirical Formula (con’t)
Created by C. Ippolito
Dec 2006