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Empirical and Molecular Formula Calculations

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Empirical formula is the smallest whole number ratio of moles of each element in a compound.

CaCl₂ --> there is 1 mole of calcium for every 2 moles of chlorine

Level 1 Empirical formula questions

What is the empirical formula of the following compounds? (so reduce the formula if you can)

Level 2 Given the percent of each element determine the empirical formal.

Step 1 assume the mass to be 100g so the percent becomes grams

Step 2 determine the moles of each element

Step 3 determine the mole ratio by dividing each elements number of moles by the smallest

Step 4 Double, triple .. to get an integer is they are not all whole numbers

example

What is the empirical formula given percent composition?

62.1% C, 5.21%H, 12.1% N, and 20.7% O

Step 1 Mass is 100% so % becomes grams

62.1g C, 5.21gH, 12.1g N, and 20.7g O

Step 2 determine the moles of each element

62.1 g C x (12.0 g C / mole) = 5.18 moles C
5.21 g H x (1.0 g H / mole) = 5.21 moles H
12.1 g N x (14.0 g N / mole) = 0.864 moles N
20.7 g O x (16.0 g O / mole) = 1.29 moles O

Step 3 determine the mole ratio by dividing each elements number of moles by the smallest

Dividing by the smallest (0.864) we get

C: 5.18 / 0.864 = 6.00
H: 5.21 / 0.864 = 6.03
N: 0.864 / 0.864 = 1.00
O: 1.29 / 0.864 = 1.49

Step 4 Double, triple .. to get an integer is they are not all whole numbers

To get all integers, double these values and you get C₁₂H₁₂N₂O₃

Level 3 Determining Molecular Formulas (a whole # multiple of the empirical)

This is actually just a little longer than Level 2. The question will also include molecular mass.

Step 1 After you determine the empirical formula, determine its mass.

Step 2 Determine how many times greater the molecular mass is compared to the mass of the empirical formula.

Step 3 Multiply the empirical formula by this number

Example-  from the percentages you get an empirical formula of CH₂ and the molecular mass is 56.0g/mol.

Step 1 After you determine the empirical formula, determine its mass.

CH₂

1 carbon 12.0 + 2 (hydrogens 1.0) =14.0g/mol

Step 2 Determine how many times greater the molecular mass is compared to the mass of the empirical formula.

molecular mass/ empirical formulas mass

56.0g/mol  / 14.0g/mol = 4

Step 3 Multiply the empirical formula by this number

4x CH₂ =C₄H₈  

_***note

_{if step 2 does not work out to be a whole number your empirical formula is wrong or your teacher screwed up}

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