Empirical and Molecular Empirical and Molecular FormulasFormulas

Empirical FormulaEmpirical Formula Empirical vs. Molecular FormulasEmpirical vs. Molecular Formulas

Molecular FormulaMolecular Formula – a formula that specifies – a formula that specifies the the actual number actual number of atoms of each element in of atoms of each element in one molecule of a compound.one molecule of a compound.

Empirical FormulaEmpirical Formula – a formula with the – a formula with the smallest whole-number mole ratio smallest whole-number mole ratio of the of the elements that make up a compound.elements that make up a compound.

Empirical FormulaEmpirical Formula Empirical FormulaEmpirical Formula

May or may not be the same as the molecular May or may not be the same as the molecular formulaformula

Molecular formula is always a simple multiple of Molecular formula is always a simple multiple of the empirical formulathe empirical formula

ex. ex. HH22OO22

Empirical formula is HOEmpirical formula is HO Molecular formula is TWO times the empirical Molecular formula is TWO times the empirical

formulaformula

How to calculate an empirical formulaHow to calculate an empirical formula How to calculate:How to calculate:

STEP 1: You will be given either masses or STEP 1: You will be given either masses or percent composition.percent composition.

STEP 2: If you are given % composition, STEP 2: If you are given % composition, turn it into grams by assuming a 100.0 g turn it into grams by assuming a 100.0 g sample. NOTE: If you are given mass, you sample. NOTE: If you are given mass, you do not need to do this step.do not need to do this step.

STEP 3: Convert the masses to the number STEP 3: Convert the masses to the number of moles of each element.of moles of each element.

STEP 4: Figure out the proportion of moles of STEP 4: Figure out the proportion of moles of each element in the compound by dividing each element in the compound by dividing each by the smallest number of moles.each by the smallest number of moles.

STEP 5: If step 4 resulted in whole numbers, STEP 5: If step 4 resulted in whole numbers, you are done! However, if there were you are done! However, if there were decimals, you will need to multiply by small, decimals, you will need to multiply by small, whole numbers until you have whole numbers.whole numbers until you have whole numbers.

An example:An example: STEP 1: STEP 1:

Compound is 40.05% S and 59.95% OCompound is 40.05% S and 59.95% O STEP 2: STEP 2:

I assume 100 g of the compound, so it is:I assume 100 g of the compound, so it is: 40.05 g S and 59.95 g O40.05 g S and 59.95 g O

STEP 3:STEP 3: 40.05 g S•(1 mol S/32.07 g S) = 1.249 mol S40.05 g S•(1 mol S/32.07 g S) = 1.249 mol S 59.95 g O•(1 mol O/16.00 g O) = 3.747 mol O59.95 g O•(1 mol O/16.00 g O) = 3.747 mol O

Continued…Continued… STEP 4:STEP 4:

1.249 mol S : 3.747 mol O1.249 mol S : 3.747 mol O Divide each by 1.249 (smallest number in ratio)Divide each by 1.249 (smallest number in ratio) 1 mol S : 3 mol O1 mol S : 3 mol O

STEP 5:STEP 5: SOSO33

You are done! The compound is sulfur You are done! The compound is sulfur trioxide.trioxide.

A way to remember those steps:A way to remember those steps: A Poem by Joel Thompson:A Poem by Joel Thompson: Percent to massPercent to mass Mass to moleMass to mole Divide by smallDivide by small Multiply ‘til wholeMultiply ‘til whole

Molecular FormulaMolecular Formula Molecular Formula Molecular Formula – this tells us – this tells us how many how many

atoms of each type there really are in the atoms of each type there really are in the compound.compound.

Can two substances have the same empirical Can two substances have the same empirical formula but be different? formula but be different? YES! YES! Benzene vs. acetylene: CBenzene vs. acetylene: C66HH66 vs. C vs. C22HH22

What is their empirical formula? How is this What is their empirical formula? How is this different from ionic compounds?different from ionic compounds?

Calculating Molecular FormulaCalculating Molecular Formula STEP 1: STEP 1:

You will be given the molar mass of the compound You will be given the molar mass of the compound and the empirical formula.and the empirical formula.

STEP 2:STEP 2: Calculate the empirical mass (mass of the empirical Calculate the empirical mass (mass of the empirical

formula). formula). STEP 3:STEP 3:

Divide the given molar mass by the empirical Divide the given molar mass by the empirical mass. You should get a small whole number.mass. You should get a small whole number.

STEP 4:STEP 4: Multiply the subscripts of the empirical formula Multiply the subscripts of the empirical formula

with the number obtained.with the number obtained.

Molecular Formula ExampleMolecular Formula Example STEP 1:STEP 1:

The empirical formula is CHThe empirical formula is CH22O and the molar mass O and the molar mass is 180 g.is 180 g.

STEP 2:STEP 2: The empirical mass is 12g + 2(1)g + 16g = 30gThe empirical mass is 12g + 2(1)g + 16g = 30g

STEP 3:STEP 3: 180g/ 30g = 6180g/ 30g = 6

STEP 4: STEP 4: CHCH22O becomes CO becomes C66HH1212OO66

Closure:Closure: Acetylene is a gas that is used as a fuel for Acetylene is a gas that is used as a fuel for

welding. Benzene is a liquid solvent. welding. Benzene is a liquid solvent. How are they similar?How are they similar? How are they different?How are they different? Why are they different? Why are they different? Why is one a gas at room temperature and one a Why is one a gas at room temperature and one a

liquid?liquid?