Specification Point 1.32:## Know what is meant by the terms Empirical Formula and Molecular Formula

**Empirical Formula & 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 H_{2}O. 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 H_{2}O

**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:**

Name of compound | Empirical formula | Molecular formula |
---|---|---|

Methane | CH_{4} | CH_{4} |

Ethane | CH_{3} | C_{2}H_{6} |

Ethene | CH_{2} | C_{2}H_{4} |

Benzene | CH | C_{6}H_{6} |

*Example:*

The Empirical Formula of X is C_{4}H_{10}S_{1} 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 = C _{8}H_{2}0S_{2}**

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.

**Metal Oxides**

*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

**Water and Salts containing Water of Crystallisation**

*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.xH_{2}O ’

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### Author: Jamie

Jamie got a First class degree in Chemistry from Oxford University before going on to teach chemistry full time as a professional tutor. He’s put together these handy revision notes to match the Edexcel IGCSE Chemistry specification so you can learn exactly what you need to know for your exams.

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