Methodology: experiment, we heated the crucible for


We can ensure fair testing by:

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Reducing random error: ensured mass of magnesium was accurate by weighing several times and obtaining an average, lifted lid every 3 mins to reduce/prevent reactants from escaping, performed on the same day in the same lab to ensure no drastic changes in atmospheric conditions, balance calibrated prior to weighing.

Reducing systematic error: heating crucible prior to the experiment, to remove any previous residue that would affect mass. Scrubbing magnesium ribbon with sand paper to remove previously oxidized magnesium.

• We can determine when the magnesium has been heated enough when there is no more change in mass.


1) Prior to beginning the experiment, we heated the crucible for 5 mins before we added the magnesium.
2) We weighed the crucible and the lid to obtain its mass.
3) We weighed out approximately 0.250g of magnesium before scrubbing with sandpaper.
4) We folded the strip of magnesium and put it in the crucible.
5) We then noted down the mass of the crucible with the lid and the magnesium inside
6) We then heated the crucible, lid and magnesium.
7) Then using tongs, we lifted the crucible lid every 2 mins for 10seconds to observe the reaction
8) After the magnesium had stopped burning (15 mins) we left the crucible to cool.
9) When it was cool enough we noted down the mass of the crucible, the lid and its contents
10) We then heated for a further 5 mins and again left it to cool.
11) After the crucible had cooled we then weighed it again.
12) This process is supposed to be continued until no more change in mass is seen

Apparatus set up:

Measurement 1 Measurement 2 Average
Mass of crucible&lid
?0.001g 22.722 22.732 22.727
Mass of crucible,lid & Mg/ ?0.001g
(before heating) 22.986 22.983 22.985
Mass of crucible,lid & Mg/ ?0.001g
(after 15 mins heating)
23.105 23.107 23.106
Mass of crucible,lid & Mg/ ?0.001g
(after 20 mins of heating) 23.111 23.118 23.115
Mass of crucible,lid & MgO/ ?0.001g
(after 25 mins of heating)
23.127 23.124 23.125
Mass of MgO 23.127-22.722=0.405 23.124-22.732=0.392 0.399
Mass of Mg 22.986-22.722=0.264 22.983-22.732=0.251 0.258
Moles of Mg 0.258/24.31=0.011
% of Mg (0.258/0.399)x100=64.66%
Mass of O2
23.127-22.986=0.141 23.124-22.983=0.141 0.141
Moles of O2 0.130/6=0.008
% Of O2 (0.141/0.399)x100=35.33%


• Until 6 mins had passed no burning or light was observed.
• After 6 mins a red light was observed which became a bright white light after the lid had had been lifted for a few seconds.
• After 10 minutes white ash with a red glow could be seen.
• After the 15 minute weigh, it was reignited with an orange glow
• There was no ignition after 20 mins, but we heated it for a further 5 mins


Mass of magnesium (g) ?0.001g Mass of MgO produced (g) ?0.001g
0.251 0.399
0.395 0.656
0.481 0.777
0.408 0.631
0.480 0.610

Percentage composition: (please see table for workings out) 64.6% magnesium (Mg) 33.35% oxygen (O)

Empirical formula:

Mass of Mg = 0.258g
Mass of magnesium oxide = mass of Mg + mass of O

0.399g = 0.258g + mass of O
Mass of O = (0.399– 0.258) g = 0.141g
Moles of Mg= 0.258g ? 24.31= 0.011 mol
Moles of O = 0.130 ? 6 = 0.008 mol

element Mass/g Moles ratio intergers
O 0.141 0.011 0.141/0.008=1.38 1
Mg 0.258 0.008 0.008/0.008=1.00 1

Therefore empirical formula = MgO


How does varying the mass of magnesium affect the mass of magnesium oxide produced?

Theoretically increasing the mass of magnesium should also increase the mass of magnesium oxide produced. However, the data that was collected was unable to be used to make a graph to prove this.

What is the percentage composition of the magnesium oxide produced in the lab?

Through calculation the percentage composition of MgO was found to be 64.6% magnesium (Mg) and 33.35% oxygen (O)

What is the empirical formula of the magnesium oxide produced in the lab?

Empirical formula of the magnesium oxide was found to be Mg1 O1 which is MgO.


There was insufficient time to fully complete the experiment and check that there was no change in mass, which has altered the accuracy of the experiment.


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