Molecular Formulas

Empirical Formula Empirical vs. Molecular Formulas Molecular Formula – a formula that specifies the actual number of atoms of each element in one molecule of a compound. Empirical Formula – a formula with the smallest whole-number mole ratio of the elements that make up a compound.

Empirical Formula Empirical Formula May or may not be the same as the molecular formula Molecular formula is always a simple multiple of the empirical formula ex. H2O2 Empirical formula is HO Molecular formula is TWO times the empirical formula

How to calculate an empirical formula STEP 1: You will be given either masses or percent composition. STEP 2: If you are given % composition, turn it into grams by assuming a 100.0 g sample. NOTE: If you are given mass, you do not need to do this step. STEP 3: Convert the masses to the number of moles of each element.

STEP 4: Figure out the proportion of moles of each element in the compound by dividing each by the smallest number of moles. STEP 5: If step 4 resulted in whole numbers, you are done! However, if there were decimals, you will need to multiply by small, whole numbers until you have whole numbers.

An example: STEP 1: STEP 2: STEP 3: Compound is 40.05% S and 59.95% O I assume 100 g of the compound, so it is: 40.05 g S and 59.95 g O STEP 3: 40.05 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 O

Continued… STEP 4: STEP 5: 1.249 mol S : 3.747 mol O Divide each by 1.249 (smallest number in ratio) 1 mol S : 3 mol O STEP 5: SO3 You are done! The compound is sulfur trioxide.

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

Molecular Formula Molecular Formula – this tells us how many atoms of each type there really are in the compound. Can two substances have the same empirical formula but be different? YES! Benzene vs. acetylene: C6H6 vs. C2H2 What is their empirical formula? How is this different from ionic compounds?

Calculating Molecular Formula STEP 1: You will be given the molar mass of the compound and the empirical formula. STEP 2: Calculate the empirical mass (mass of the empirical formula). STEP 3: Divide the given molar mass by the empirical mass. You should get a small whole number. STEP 4: Multiply the subscripts of the empirical formula with the number obtained.

Molecular Formula Example STEP 1: The empirical formula is CH2O and the molar mass is 180.18 g. STEP 2: The empirical mass is 12.01g + 2*1.01g + 16.00g = 30.03 g STEP 3: 180.18 g/ 30.03 g = 6 STEP 4: CH2O becomes C6H12O6

Closure: Acetylene is a gas that is used as a fuel for welding. Benzene is a liquid solvent. How are they similar? How are they different? Why are they different? Why is one a gas at room temperature and one a liquid?