Molecular formulas and Ionic Bonding

Molecular formulas and Ionic Bonding
Molecular Formulas 5.1, 5, 8-10
Empirical formula
Percent composition
Ionic Bonding 5.3-4
Lattice energy 10.8
Naming Compounds 5.6-7
Mary J. Bojan
Chem 110
1
Atoms (or ions) combine (bond) to form
molecules (or compounds).
Each molecule has a chemical formula.
The chemical formula indicates
– which atoms are found in the molecule, and
– in what proportion they are found.
MJ Bojan
Chem 110
2
CHEMICAL FORMULA: gives the TOTAL number
of elements in a molecule or formula unit.
EMPIRICAL FORMULA: Chemical formula with the smallest
integer subscripts for a given composition.
MJ Bojan
Chem 110
3
To get the molecular formula, we need
more information: Molecular weight
Mass Spectroscopy is used to get
atomic and molecular weights.
MOLAR MASS or FORMULA WEIGHT sum of atomic
weights
The connection between experimental property (mass) and moles:
C8H18
MJ Bojan
8C
18H
12.01
1.008
96.08
18.14
Chem 110
4
What does a formula tell you?
How many atoms are in the formula Mg3(PO4)2
a) 3
b) 5
c) 10
d) 13
How many moles of Mg3(PO4)2 are in a 10.0 g sample?
a) 0.0380 moles
b)  0.0595 moles
c)  0.0431 moles
MJ Bojan
Chem 110
5
What does a formula tell you?
How many atoms are in 10.0 g of Mg3(PO4)2?
a)
b)
c) 
d) 
2.59 × 1022 atoms
3.58 × 1022 atoms
2.29 × 1022 atoms
2.97 × 1023 atoms
MJ Bojan
Chem 110
6
A HYDRATE is a salt that has water loosely associated with it.
Dehydration means to drive off the water.
If Epsom salts, MgSO4 Ÿ x H2O, is heated to 300° C, all of the water of
hydration is lost. After 1.687g sample of the hydrate is heated, 0.824g of
MgSO4 remains. How many molecules of water are there per formula
unit of MgSO4?
MJ Bojan
Chem 110
7
Empirical Formula from Analysis
How do we determine the empirical formula?
EXPERIMENT!
•  Find mass of each component in a sample.
•  Use ____________ to find mole ratio of atoms.
•  Mole ratio gives empirical formula
•  Molecular weight (obtained from a separate experiment)
can be used to get ___________________
MJ Bojan
Chem 110
8
The analysis of a 200 mg tablet of ibuprofen has 151.4 mg of
carbon, 17.6 mg of hydrogen and 31.0 mg of oxygen. The
molecular weight of ibuprofen is 206.29 g/mol.
What are the empirical and molecular formulas of ibuprofen?
MJ Bojan
Chem 110
9
PERCENT COMPOSITION is the % mass of
each element in a compound.
If empirical formula is known, % mass of each element
can be determined.
(atoms of element)(AW)
% of element =
! 100%
FW of compound
What is the mass percent of C in CO2?
C = 12.01 amu O = 16.0 amu
FW =12.01 + 2(16.0) = 44.01 amu
MJ Bojan
Chem 110
10
PERCENT COMPOSITION can also be defined as
the mass of an element in a 100 g sample.
mass of element
% of element =
! 100%
100g sample
I have 2.0 g of a sample that is 54.2% C by mass. How
many grams of C are in the sample?
MJ Bojan
Chem 110
11
Given the % composition, Empirical Formula can
be determined
An analysis of nicotine, a poisonous compound found in
tobacco leaves, shows that it is 74.0%C, 8.65% H and
17.35% N. Its molar mass is 162 g/mol.
What are the empirical and molecular formulas of nicotine?
MJ Bojan
Chem 110
12