Magnesium Oxide

Formula: MgO
Equation: 2Mg (s) + O₂ (g) → 2MgO (s)
Observation: A bright, white light + white solid powder produced

Example:

- A student heated up some magnesium ribbon. It burns in the air, combining with oxygen to make magnesium oxide.

Results:

Mass of crucible + lid = 25.00 g
mass of crucible + lid + Mg (before heating) = 25.24 g
mass of crucible + lid + Mg (after heating) = 25.40 g

Step 1:

Step 3: Mg : O
0.01 : 0.01
1 : 1 formula = MgO

Method:

1. Put on eye protection.
2. Weigh a clean, dry crucible and lid (A).
3. Clean a 10 cm strip of magnesium ribbon with emery paper (to rub off the oxygen level). Make the magnesium coil and place it in the crucible, lid + Mg (B).
4. Set up apparatus (shown on next pg) and heat crucible strongly. From time to time, lift the crucible lid up (a small distance), then replace it. Try to avoid any loss of white smoke.

5. When there is no further sign of a reaction, allow the crucible lid to cool. Then, re-weigh the crucible lid until the mass remains constant.

- We had to lift the lid slightly to let oxygen in.
- It was important to avoid losing the white smoke to minimise (loss of MgO).

Sources of Error:

- Lifting lid too long; solid escapes → goal mass ratio = lower
- Not listing for enough/incomplete reaction → Mg would be higher in ratio, and O would be lower.
- This results in a less accurate balance.
- Not cleaning the Mg piece properly → less Mg oxidised → less produced → MgO ratio = lower

Decomposing

- Sodium hydrogen carbonate thermal decomposition: breaking things down using heat.

Method:

- Weigh an empty crucible + lid.
- Weigh exactly about 5 g sodium hydrogen carbonate (NaHCO₃) into a crucible; record the new mass of the crucible + lid + contents.
- Heat the crucible strongly using a roaring Bunsen flame for 10 minutes.
- Cool for 10 minutes, then reweigh the crucible, lid, and solid product.

Example Results (Masses):

M1: empty crucible + lid = 25.95 g
M2: empty crucible + NaHCO₃ (before heating) = 30.95 g
M3: empty crucible + lid + product (after heating) = 29.08 g

m (NaHCO₃) used → M2 - M1: 5 g
mass of solid product formed: 3.13 g

- Using the mass of (actual) NaHCO₃ used in the experiment, calculate the mass of the solid product expected after heating. Prove the equation is right.

- 3.15g is very close to 3.18g, so the equation is correct.

- Make sure you heat the sodium hydrogen carbonate to a constant mass (cool the substance down, weigh it, and repeat) to ensure the reaction has completed and it has broken down fully.

- One major source of error = Incomplete decomposition → stopping when the reaction is not complete is a common mistake.

- Repeating data many times can improve its accuracy.