1. How do chemists determine the formula of compounds?

2. 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?

3. Chemical Formulas of Compounds
What do formulas tell us?
NO 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 x1023 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.

4. 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
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.
We will come to this later

5. 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?

6. 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.
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

7. A sample of a brown gas, a major air pollutant, is found to contain 2
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 1 mol N = moles of N
14.01 g N
5.34 g of O x mol O = moles of O
16.00 g of O
Formula:

8. 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?

9. Example
A sample of urea is decomposed into its elements. The following products are measured:
nitrogen: g
hydrogen: g
carbon: g
oxygen: g

10. Steps in determining the formula of urea
Find the number of moles of each element present:
nitrogen: g x ( )=
hydrogen: g X ( )=
carbon: g X ( )=
oxygen: g X ( )=

11. Empirical Formula from % Composition
A substance has the following composition by mass: % Na ; % B ; % H
What is the empirical formula of the substance?
1) Consider a sample size of 100 grams
This will contain g of Na, grams of B and grams H
2) Determine the number of moles of each
3) Determine the simplest whole number ratio

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