1. Different Types of Formulas
Molecular Formula – shows # of atoms in one molecule or formula unit
Empirical Formula – shows smallest whole number mole ratio
Structural Formula- molecular formula info PLUS bonding electrons
and atomic arrangement
C6H6 CH
All different types of formulas for the same compound…benzene!

2. Finding an Empirical Formula from Experimental Data
1. Find mass of each element (grams)
2. Find mols of each element (convert from grams to mols)
3. Divide each “# of mol” by the smallest “# of mol” (ratios)
4. Write out formula with ratios from step 3 as subscripts
(multiply by smallest integer possible for whole #’s if needed)
PRACTICE... A compound is 45.5% yttrium and 54.5% chlorine. Find its empirical formula.
YCl3

3. A ruthenium/sulfur compound is 67.7% Ru. Find its empirical formula.
RuS1.5
Ru2S3

4. (Empirical Formula)n = Molecular Formula
To find molecular formula…
A. Find empirical formula.
B. Find mass of
empirical formula.
C. Find n = Molar Mass molecular
empirical mass
D. Multiply all parts of
empirical formula by n.
“How many empiricals fit into the molecular?”

5. A carbon/hydrogen compound is 7.7% H and has a
molar mass of 78 g. Find its molecular formula.
emprical formula = CH
Empirical
mass =
78 g
13 g
n =
= 6
C6H6
13 g

6. 92 g 46 g MMempirical = 46 g n = = 2 N2O4 ÷ 1.879 ÷ 1.879
A compound has g nitrogen, g oxygen,
and molar mass 92 g. Find molecular formula.
÷ 1.879
÷ 1.879
Empirical formula = NO2
92 g
46 g
MMempirical =
46 g
n =
= 2
N2O4

7. QUICK SUMMARY the mole is a chemist’s bridge out of the atomic world (amu) into the real world (g)!
Atomic Mass vs Molar Mass
6.022x1023 particles
H2 = _______
H2O = ________
MgSO4 = ________
(NH4)3PO4 = ________
2 amu
18 amu
120 amu
149 amu
H2 = _____
H2O = _____
MgSO4 = _____
(NH4)3PO4 = _____
2 g
18 g
120 g
149 g

8. LET’S REVIEW… 6.02 x 1023 100 3 2 1 # of O atoms # of H atoms
# of H2O molecules 2 1 4 2 6 3
200
100
2 (6.02 x 1023)‏
6.02 x 1023
molar mass:
the mass of one mole of a substance
18.0 g/mol
2.0 g/mol
16.0 g/mol

9. Nitric Acid MOLAR MASS PRACTICE… Ammonium Phosphate # # (NH4)3PO4 HNO3
Element
MM (g)
Total (g) 1 H
1.01 N
14.01 3 O
16.00
48.00
TOTAL MOLAR MASS
63.02 #
Element
MM (g)
Total (g) 3 N
14.01
42.03 12 H
1.01
12.12 1 P
30.97 4 O
16.00
64.00
TOTAL MOLAR MASS
149.12
Nitric Acid
Ammonium Phosphate

10. percentage composition: the mass % of each element in a compound
mass % of element =
g element
molar mass of compound
x 100
Find the % composition of ammonium phosphate.
(NH4)3PO4
42.03 g N : g
= % N
12.12 g H : g
= % H
30.97 g P : g
= % P
64.00 g O : g
= % O

11. Review Problems
A g sample of a technetium/oxygen compound
contains g of Tc. Find the empirical formula.
TcO3.5
Tc2O7

12. n = Review Problems A compound contains 70.35 g C and 14.65 g H.
Its molar mass is 58 g. Find its molecular formula.
emp. form.  C2H5
58 g
n =
MMemp =
29 g
= 2
C4H10
29 g

13. ? PbN4O8 ? Pb(NO2)4 Pb? 4 NO21– Review Problems
A compound contains 4.63 g lead, 1.25 g nitrogen,
and 2.87 g oxygen. Name the compound. ?
PbN4O8 ?
lead (IV) nitrite
Pb(NO2)4
(plumbic nitrite)
Pb? 4 NO21–