1. AP CHEMISTRY NOTES
Ch 3 Stoichiometry

2. 3.1 Counting by Weighing Find the average mass
= total mass of substance / number of substance

3. 3.2 Atomic Masses Find the average atomic mass
= total % of amu of isotopes
Average Atomic mass
Ex: Average atomic mass of carbon 98.89%x12amu+1.11%x13amu=12.01amu
(Given atomic mass)

5. 3.3 The mole Abbreviated mol Avogadro’s number Conversion factor
6.022 x 1023
Conversion factor
Ex. Carbon
6.022 x 1023 atoms)(12 amu/atom) = 12g
6.022 X 1023 amu = 1g

6. Molecule Mole Atoms 1 mole= 6.022x1023 particles
1 molecule= ____ atoms
Atoms

8. 3.4 Molar Mass Calculating molar mass
Molar mass and numbers of molecules

9. Mass Atoms Mole Particle 1 mole= 6.022x1023 particles
1 mole= molar mass
Mass
Mole
Particle
1 molecule= ____ atoms
Atoms

10. 3.5 Percent Composition of Compounds
Mass percent 1 (from the formula)
= Mass of an element in a compound
mass of 1 mol of compound
Mass percent of C, H, O in C2H5OH

11. 3.5 Percent Composition of Compounds
Try Ex 3.9
Mass percent of C, H, O in C2H5OH
Mass of C in 1 mol C2H5OH x 100% =
Mass of 1 mol C2H5OH
Mass of H in 1 mol C2H5OH x 100% =
Mass of O in 1 mol C2H5OH x 100% =
52.14%
13.13%
34.73%
Add up %

12. 3.6 Determining the Formula of a Compound
From an experiment to find mass of C, H, O

13. Unknown Compound (0.1156g) with C, H, N
CO2 : g (all C converted to CO2)
H2O : g (all H converted to H2O
Mass of C in a compound
= g CO2 x g C =
44.01 g CO2
Find mass % of C
g C x 100% =
0.1156g compound
g C
38.67 %

14. Mass % of H CO2 : 0.1638g (all C converted to CO2)
H2O : g (all H converted to H2O
Mass of H in a compound
= g H2O x g H =
18.02 g H2O
Find mass % of H
g H x 100% =
0.1156g compound
g H
16.22 %

15. Mass % of N Find mass % of N 100.00% - (% of C + % of H)
100.00% - (38.67% %) =
Assume there is a g compound
Mass of carbon
Mass of hydrogen
Mass of nitrogen
45.11 %N
38.67 g
16.22 g
45.11 g

16. Moles of C, H, N 38.67 g C x 1 mol C = 12.01 g C 16.22 g H x 1 mol H =
45.11 g N x 1 mol N =
14.01 g N
3.220 mol C
16.09 mol H
3.219 mol N

17. Smallest whole number ratio of atoms
divide by the smallest mol
C: 3.220mol =
3.220mol
H: mol H =
N: mol N =
1.000 = 1
4.997 = 5
1.000 = 1

18. Determine the formula CH5N as an empirical formula
C2H10N2, C3H15N3 might be a molecular formula
Represented by (CH5N)n

19. Empirical Formula Determination
Since mass percentage gives the number of grams of a particular element per 100 grams of compound, base the calculation on 100 grams of compound. Each percent will then represent the mass in grams of that element.
Determine the number of moles of each element present in 100 grams of compound using the atomic masses of the elements present.

20. Empirical Formula Determination
Divide each value of the number of moles by the smallest of the values. If each resulting number is a whole number (after appropriate rounding), these numbers represent the subscripts of the elements in the empirical formula.
If the numbers obtained in the previous step are not whole numbers, multiply each number by an integer so that the results are all whole numbers.

21. Molecular Formula Determination-1
Obtain the empirical formula.
Compute the mass corresponding to the empirical formula.
Calculate the ratio Molecular mass
Empirical formula mass
The integer from the previous step represents the number of empirical formula units in one molecule. When the empirical formula subscripts are multiplied by the integer, the molecular formula results.
Molecular formula = (empirical formula) x molar mass                      empirical formula mass

22. Molecular Formula Determination-2
Using the mass percentages and the molar mass, determine the mass of each element present in one mole of compound.
Determine the number of moles of each element present in one mole of compound.
The integers from the previous step represent the subscripts in the molecular formula.

23. Pg 119 problems 70 72 74 76 78 80

24. 3.7 Chemical equation Chemical equation Reactants Products States
The atoms have been reorganized
Bonds have been broken
New ones have been formed
Reactants
Products
States
(s), (l), (g), (aq)

25. 3.8 Balancing Chemical Equations4
_ NH3(g)+_O2(g)_NO(g)+_H2O(g) 5 4 6