Specification Point 1.31:## Understand how the formulae of simple compounds can be obtained experimentally, including Metal Oxides, Water and Salts containing Water of Crystallisation

**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 ’

Specification Point 1.36:## Practical: Know how to determine the formula of a Metal Oxide by Combustion (E.g. Magnesium Oxide) or by Reduction (E.g. Copper (II) Oxide)

**Combustion of Metal Oxides**

**Metal Oxide: **When a metal reacts with and gains oxygen

*The apparatus needed to find the formulae of **a Metal Oxide by combustion*

**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 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 a b

Mole a / M_{r} b / M_{r}

= x = y

Ratio x : y

**Step 3 – **Represent the Ratio into the Form ‘ xO ‘ E.g, MgO

**Reduction of Metal Oxides**

*The apparatus needed to find the formulae of **a Metal Oxide by reduction*

**Method:**

- Measure mass of metal oxide
- Place metal oxide into a horizontal boiling tube held by a clamp and heat using bunsen burner
- Heat until metal oxide completely changes colour, meaning that all the Oxygen has been reduced
- Measure mass of the remaining metal powder

**Working out Empirical Formula / Formulae:**

** Mass of Metal:** Measure Mass of the Remaining Metal Powder

** Mass of Oxygen:** Subtract Mass of the Remaining Metal Powder from the Mass of Metal Oxide

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

**Step 2 – **Simplify the Ratio

**Metal** **Oxygen**

Mass a b

Mole a / M_{r} b / M_{r}

= x = y

Ratio x : y

**Step 3 – **Represent the Ratio into the Form ‘ xO ‘ E.g, MgO

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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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