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 Chemical Formulas of CompoundsCompounds

• What do formulas tell us?What do formulas tell us?

• NONO22 1 atom of N for every 2 atoms of O 1 atom of N for every 2 atoms of O • 5 atoms of N for every 10 atoms of O 5 atoms of N for every 10 atoms of O • 100 atoms of N for every 200 atoms of O100 atoms of N for every 200 atoms of O• 6.022x106.022x102323 atoms of N for every 12.044x10 atoms of N for every 12.044x102323 atoms of O atoms of O • This is the same as: This is the same as:

• 1 mole of N : 2 moles of O atoms 1 mole of N : 2 moles of O atoms • Formulas give the relative numbers of atoms or Formulas give the relative numbers of atoms or

moles of each element in a formula unit - always a moles of each element in a formula unit - always a whole number ratio (the law of definite whole number ratio (the law of definite proportions). proportions).

• If we know or can determine the relative number of If we know or can determine the relative number of moles of each element in a compound, we can moles of each element in a compound, we can determine a formula for the compound.determine a formula for the compound.

Types of FormulasTypes of Formulas

• Empirical FormulaEmpirical Formula

The formula of a compound that expresses the The formula of a compound that expresses the smallest whole number ratiosmallest whole number ratio of the atoms present. of the atoms present.

OR theOR the LOWEST WHOLE NUMBER RATIO OF MOLESLOWEST WHOLE NUMBER RATIO OF MOLES

– Ionic formula are always empirical formulaIonic formula are always empirical formula

– Molecular FormulaMolecular Formula

The formula that states the The formula that states the actualactual number of each number of each kind of atom found in kind of atom found in one moleculeone molecule of the compound. of the compound.

We will come to this later

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 To obtain an Empirical Empirical FormulaFormula

1.1. Determine the mass in grams of each Determine the mass in grams of each element present, if necessary.element present, if necessary.

2.2. Calculate the number of Calculate the number of molesmoles of of each each element.element.

3.3. Divide each by the smallest number of moles Divide each by the smallest number of moles to obtain the to obtain the simplest whole number ratio.simplest whole number ratio.

4.4. If whole numbers are not obtainedIf whole numbers are not obtained** in step in step 3), multiply through by the smallest number 3), multiply through by the smallest number that will give all whole numbersthat will give all whole numbers

** Be careful! Do not round off numbers prematurelyBe careful! Do not round off numbers prematurely

A sample of a brown gas, a major air pollutant, is A sample of a brown gas, a major air pollutant, is found to contain 2.34 g N and 5.34g O. Determine a found to contain 2.34 g N and 5.34g O. Determine a formula for this substance.formula for this substance.

require require molemole ratios so convert grams to moles ratios so convert grams to moles

2.34g of N x 1 mol N 2.34g of N x 1 mol N = 0.167 moles of N= 0.167 moles of N

14.01 g N14.01 g N

5.34 g of O x 1 mol O 5.34 g of O x 1 mol O = 0.334 moles of O = 0.334 moles of O

16.00 g of O16.00 g of O

Formula:Formula:

0.334 0.167ON 0.167 0.334 2

0.167 0.167

N O NO

Calculation of the Molecular FormulaCalculation of the Molecular Formula

A compound has an empirical formula A compound has an empirical formula of NOof NO22. The colourless liquid, used in . The colourless liquid, used in

rocket engines has a molar mass of rocket engines has a molar mass of 92.0 g/mole. What is the 92.0 g/mole. What is the molecular molecular formula formula of this substance?of this substance?

ExampleExample

• 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 ( )=

hydrogen: 0.161 g X ( )=

carbon: 0.480 g X ( )=

oxygen: 0.640 g X ( )=

gN

moleN

01.14

1

gH

moleH

01.1

1

gC

moleC

01.12

1

gO

moleO

00.16

1

Empirical Formula from % Empirical Formula from % CompositionComposition

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?

1) Consider a sample size of 100 grams This will contain 60.80g of Na, 28.60

grams of B and 10.60 grams H2) Determine the number of moles of each3) Determine the simplest whole number

ratio

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