Download How do chemists determine the formula of compounds?

How do chemists determine
the formula of compounds?
What is the formula of iron oxide?
• Remember the steel wool lab?
• How would you experimentally determine the
formula?
• Do you know the mass of iron used? (g Fe)
• Do you know the mass of oxygen used? (g O)
• Could you convert both grams to moles?
• Could you find the molar ratio of Fe to O? What
would that tell you about the formula?
Chemical Formulas of Compounds
• What do formulas tell us?
• NO2 1 atom of N for every 2 atoms of O
•
•
•
•
5 atoms of N for every 10 atoms of O
100 atoms of N for every 200 atoms of O
6.022x1023 atoms of N for every 12.044x1023 atoms of O
This is the same as:
• 1 mole of N : 2 moles of O atoms
• Formulas give the relative numbers of atoms or
moles of each element in a formula unit - always a
whole number ratio (the law of definite
proportions).
• If we know or can determine the relative number of
moles of each element in a compound, we can
determine a formula for the compound.
Types of Formulas
• Empirical Formula
The formula of a compound that expresses the
smallest whole number ratio of the atoms present.
OR the LOWEST WHOLE NUMBER RATIO OF MOLES
We will come to this later
– Ionic formula are always empirical formula
– Molecular Formula
The formula that states the actual number of
each kind of atom found in one molecule of the
compound.
What if I had Manganese?
• What are the possible formulas for
manganese chloride?
• Which one will you make if you add
chloride ions to Mn metal?
To obtain an Empirical Formula
1. Determine the mass in grams of each
element present, if necessary.
2. Calculate the number of moles of each
element.
3. Divide each by the smallest number of
moles to obtain the simplest whole
number ratio.
4. If whole numbers are not obtained* in
step 3), multiply through by the smallest
number that will give all whole numbers
* Be
careful! Do not round off numbers prematurely
A sample of a brown gas, a major air pollutant,
is found to contain 2.34 g N and 5.34g O.
Determine a formula for this substance.
require mole ratios so convert grams to moles
2.34g of N x
5.34 g of O x
1 mol N
14.01 g N
1 mol O
= 0.334 moles of O
16.00 g of O
Formula: N O
0.167
= 0.167 moles of N
0.334
N 0.167 O 0.334  NO 2
0.167
0.167
Calculation of the Molecular Formula
A compound has an empirical formula
of NO2. The colourless liquid, used in
rocket engines has a molar mass of
92.0 g/mole. What is the molecular
formula of this substance?
Example
• A sample of urea is decomposed into its
elements. The following products are
measured:
nitrogen: 1.121 g
hydrogen: .161 g
carbon:
.480 g
oxygen:
.640 g
Steps in determining the formula of
urea
1. Find the number of moles of each
element present:
nitrogen: 1.121 g x ( 1moleN )=
14.01gN
1moleH
1.01gH
hydrogen: 0.161 g X (
)=
1moleC
carbon: 0.480 g X ( 12.01gC )=
oxygen: 0.640 g X ( 1moleO )=
16.00 gO
2. Find the lowest whole number ratio in
moles (the empirical formula)
• The subscripts in the formula will reflect
the smallest whole number ratio between
the moles of each of the elements present.
Empirical Formula from % Composition
A substance has the following composition by
mass: 60.80 % Na ; 28.60 % B ; 10.60 % H
What is the empirical formula of the substance?
Consider a sample size of 100 grams
This will contain 28.60 grams of B and
10.60 grams H
Determine the number of moles of each
Determine the simplest whole number ratio
end
Another example
• When 2.30 g of steel wool (mostly iron) is
burned, a white ash is produced with a
mass of 3.29 g. Assuming that the gray
ash is an oxide of iron, what is the formula
of this new substance?
1. Determine the number of moles
of iron and oxygen are present
2. Find the smallest whole number
ratio of the moles of iron and
oxygen
3. Write a formula which reflects the
smallest whole number ratio between the
atoms of iron and oxygen.
Percent Composition
What is the percent carbon in C5H8NO4 (the
glutamic acid used to make MSG
monosodium glutamate), a compound used
to flavor foods and tenderize meats?
a) 8.22 %C
b) 24.3 %C
c) 41.1 %C