1. Dr. Chirie Sumanasekera
Chapter 3:
Stoichiometry
3.6.6 Percent composition (revision)
3.6.7 Empirical formula
3.6.8 Molecular formula
AP Chemistry _ Notes
Dr. Chirie Sumanasekera
10 /31/ 2017

2. 3.6.6 Percent composition
How much of each element or polyatomic ion is present in a compound on a percent basis.
% of element = (atomic mass of element) (# of atoms in formula) x100
Molar mass of compound
STEP-1: find grams of each In 100 g of compound :
STEP-2: find % of each using the gram amount of each in 100g of compound:

3. 3.6.6 Percent composition-problems
Q1: what is the percentage of Fe, O, H and OH- ion in Fe(OH)3?
Fe = g
O = 16 g
Fe(OH)3 = g
Molar masses:
OH- = g
H = g
Fe: O: H: OH- = 1:3:3:3
(a) (b) (c) (d)
Molar ratios in 1 mol of Fe(OH)3 -->
STEP-1: g of each In 100 g of Fe(OH)3:
1 mol Fe(OH)3
107.9 g Fe(OH)3
1 mol Fe
1 mol Fe(OH)3
55.46 g Fe
1 mol Fe
(a) g Fe = 100g Fe(OH)3
= g Fe
(b) g O = 44.9g
(c) g H = 2.8 g
(d) g OH- = g
STEP-2: 100% of each In 100 g of Fe(OH)3:
55.46 g Fe
100g Fe(OH)3
(A) %Fe =
x 100
= 52.2% Fe
(B) % O = 44.9%
(C) % H = 2.8 %
(D) % OH- = %

4. 3.6.7 Empirical formula
Simplest ratio of atoms in a compound. This formula is also used to represent ionic compounds (ex. H+Cl-, Na+Cl-)
Molecular formula
Empirical formula
1. Benzene
C6H6 CH
2. Glucose
C6H12O6
CH2O
3. Butyric acid
HC4H7O2 (C4H8O2)
C2H4O
Calculations:
Find the mole ratio of atoms in the compound
Divide the number of moles of each element by the smallest mole value from 1.
If amount of elements in the compound is given in grams, you need to convert grams into moles.
If percentages of elements in a compound is given, then convert % in to grams….(if you are given only the mass of the compound, you can still find the mass of each element using molar mass of elements…
Approximate decimal values to nearest whole numbers

5. 3.6.8 Empirical formula
Q1: what is the empirical formula of a compound that contains 4.0 g of calcium,7.1 g of Chlorine?
Ca =40.08 g/mol Cl =35.45g/mol
? mol of Ca = 4g Ca
1 mol Ca
40.08 g Ca
= 0.10 mol Ca
? mol of Cl = 7.1g Cl
1 mol Cl
35.45 g Cl
= 0.20 mol Ca
STEP-1:Find the mole ratio of atoms in the compound
Empirical formula = CaCl2
1 mol Ca
0.10
0.2 mol Cl
= 2 mol Cl
STEP-2: Divide the number of moles of each element by the smallest mole value from 1

6. 3.6.8 Molecular formula
Could be the same as empirical formula or be a whole number multiple of empirical formula.
Molar mass = smallest whole number
Empirical formula
Calculations:
Find the empirical formula
Molar mass of the compound is used to determine the molecular formula: Divide the molecular mass by empirical formula mass.
Find the approximate whole number value. And multiply the empirical formula by that number
Q1: A compound has an empirical formula CH2O and has a molar mass of 180g/mol. Find its molecular formula.
empirical formula mass =12+ (1x2)+ 16 = 30g
180g/mol = 6 empirical formula units/mole
30g
= 6 (CH2O) = C6H12O6