Empirical & Molecular Formulae

Specification Point 1.32:

Know what is meant by the terms Empirical Formula and Molecular Formula

Empirical Formula: Gives the simplest whole number ratio of atoms of each element in the compound

• Calculated from knowledge of the ratio of masses of each element in the compound

Example:

A compound that contains 10 g of Hydrogen and 80 g of Oxygen has an Empirical Formula of H₂O. This can be shown by the following calculations:

Amount of Hydrogen Atoms = Mass in grams ÷ A_r of Hydrogen = (10 ÷ 1) = 10 moles

Amount of Oxygen Atoms = Mass in grams ÷ A_r of Oxygen = (80 ÷ 16) = 5 moles

The Ratio of Moles of Hydrogen Atoms to Moles of Oxygen Atoms:

                      Hydrogen            Oxygen

Moles                 10             :            5

Ratio                   2              :            1

Since equal numbers of Moles of Atoms contain the same number of atoms, the Ratio of Hydrogen Atoms to Oxygen Atoms is 2: 1

Hence the Empirical Formula is H₂O

Molecular Formula: Gives the exact numbers of atoms of each element present in the formula of the compound

• Divide the relative formula mass of the molecular formula by the relative formula mass of the empirical formula
• Multiply this to each number of elements

Relationship between Empirical and Molecular Formula:

Example:

The Empirical Formula of X is C₄H₁₀S₁ and the Relative Formula Mass of X is 180

What is the Molecular Formula of X?

Relative Formula Mass:       Carbon : 12      Hydrogen : 1      Sulfur : 32

Step 1 – Calculate Relative Formula Mass of Empirical Formula

               ( C x 4 ) + ( H x 10 ) + ( S x 1)    =   ( 12 x 4 ) + ( 1 x 10 ) + ( 32 x 1)   =   90

Step 2 – Divide Relative Formula Mass of X by Relative Formula Mass of Empirical

              Formula

               180 / 90 = 2

Step 3 – Multiply Each Number of Elements by 2

              ( C _{4 x 2}  ) + ( H _{10 x 2} ) + ( S _{1 x 2} )     =    ( C ₈ ) + ( H ₂₀ ) + ( S ₂ )

Molecular Formula of X = C₈H₂0S₂

Specification Point 1.33:

Calculate Empirical and Molecular Formulae from Experimental Data

• Find number of moles by dividing mass by relative formula mass
• Find ratio of moles
• Gives empirical formula
• To find molecular formula divide relative formula mass given by relative formula mass of empirical formula.

The apparatus needed to find the formulae of a Metal Oxide

Method:

• Measure mass of crucible with lid
• Add sample of metal into crucible and measure mass with lid (calculate the mass of metal by subtracting the mass of empty crucible)
• Strong heat the crucible over a Bunsen burner for several minutes
• Lift the lid frequently to allow sufficient air into the crucible for the metal to fully oxidise without letting magnesium oxide escape
• Continue heating until the mass of crucible remains constant (maximum mass), indicating that the reaction is complete
• Measure the mass of crucible and contents (calculate the mass of metal oxide by subtracting the mass of empty crucible)

Working out Empirical Formula / Formulae:

Mass of Metal: Subtract mass of crucible from metal and mass of empty crucible

Mass of Oxygen: Subtract mass of metal used from the mass of magnesium oxide

STEP 1 – Divide Each of the two masses by the relative atomic masses of elements

STEP 2 – Simplify the ratio

                                    Metal            Oxygen

              Mass               x                       y

              Mole              x / M_r              y / M_r

                                     = a                   = b

              Ratio                a          :           b

STEP 3 – Represent the Ratio into the ‘ Metal O ‘ E.g, MgO

The apparatus needed to find the formulae of crystals

Method:

• Measure mass of evaporating dish
• Add a known mass of hydrated salt
• Heat over a Bunsen burner, gently stirring, until the blue salt turns completely white, indicating that all the water has been lost
• Record the mass of the evaporating dish and contents

Working out Empirical Formula / Formulae:

Mass of White Anhydrous Salt: Measure Mass of White Anhydrous Salt Remaining

Mass of Water: Subtract Mass of White Anhydrous Salt Remaining from the Mass of Known Hydrated Salt

STEP 1 – Divide Each of the Two Masses by the Relative Atomic Masses of Elements

STEP 2 – Simplify the Ratio of Water to Anhydrous Salt

Anhydrous Salt      Water

Mass               a                       b

Mole              a / M_r                b / M_r

= y                    = x

Ratio                1          :           x

STEP 3 – Represent the Ratio into ‘ Salt.xH₂O ’