Pre-IB Chemistry Laboratory Activity 4


Pre-IB Chemistry Laboratory Activity 4 : Laboratory Report
Name : Lau Chun Chi, Drusus Date : 11-10-2017

Aim

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To determine the empirical formula of magnesium oxide
T o find the theoretical and actual yields of MgxOy

Background

The empirical formula of a compound is the simplest vale of the ratio of atoms of each element in the compound. Magnesium is reacted with oxygen from the air in the crucible, by heating with a bunsen burner, then the masses before and after the reaction will be recorded. At last, calculate the experimental empirical formula.

Chemical and Apparatus

Magnesium ribbon (15cm x 2)
Crucible with lid x 2
Pipeclay triangle
Crucible tongs
Heat proof mat
Tripod
Sand paper
Electronic balance
Stopper
Bunsen burner

Safety
– Wear safety googles and laboratory coat
Do not look at the burning magnesium directly as it’s given out strong light
Do not touch any hot apparatus and equipment

Procedure
Weigh a clean, dry crucible and lid.
Clean a magnesium ribbon with sand paper.
Coil the magnesium ribbon around a pen and put into the crucible.
Cover the crucible with lid and reweigh.
Heat the crucible with non-luminous flame as shown on the left. Lift the crucible so the top of the cone is in contact with the surface of the crucible until the bottom of the crucible glows red.
After the crucible glows,
lift the lid slightly with crucible tongs
Lift the lid slightly every 30second for 10 minutes.
Heat the crucible for 1 more minute
Allow the crucibe cool down
Record all the result.
Repeat the step (1-10).
Result

Experiment 1

Observation (Qualitative Data)

Processed Data

Conclusion

Evaluation

The main observation

While the crucible was burning by a non-luminous flame on Pipeclay triangle, the base of the crucible turned red. When we lifted the lid in order to let air goes in to the crucible, some white smoke were releasing from the crucible at the same time. After 8 minutes, we tried to peek into the crucible and what we saw were the magnesium become red, producing strong white light and creating white smoke. As we saw this happened, we then turned the flame into a luminous flame. Lastly, about 2-3 minutes, we tried to peek into the crucible again. This time we had discovered the magnesium ribbon had turned into a bunch of white powder. After letting all equipment to cool down, we have tried to use a pencil to stir magnesium oxide, and we have discovered some magnesium ribbon didn’t turn into magnesium oxide, which we saw there were some white powder and mental.

Possible sources error

There are lots of error in experiment, no matter they are random or systematic error. Therefore, there are both random and systematic error exist in my experiment.
For random error :
When we are doing step (2), we may not fully clean the magnesium with sand paper which we are taking the measurement with some magnesium oxide included as it’s a high reactive element.
When we are doing step (7), we may lift the lid for a long time and high which led to escaping of the white smoke formed. Therefore, we could not have accurate and essential result / measurement. Secondly, we need not lift the lid frequently such as 5 minutes once instead of 30 second which led to not enough oxygen for magnesium to take chemical reaction. Thirdly, we mayn’t complete the experiment such as not all of the magnesium ribbon is turn into magnesium oxide, some are magnesium and some are magnesium oxide.
For systematic error :
We should decide a better experiment, we should use a larger mass of magnesium ribbon to do instead of 0.18g to 0.30g, because when ever the electric balance is weighing wrongly such as ± 0.01 , then overall result will become very different, because the uncertainly will become very large.

Suggest way to reduce the error

When doing step (2), we must fully clean the magnesium ribbon by sand paper,
When doing step (7), we have to lift the lid SLIGHTLY in order to reduce the chance of escaping of the white smoke formed.
When doing step (8), we should heat for a longer time until the magnesium is completely turn into magnesium oxide.
Repeat the steps (1-10) serval times.

The purposes of procedures

Step 1 : Why are the crucible and lid heated at the beginning of the experiment must be weighted ?
The crucible must be weight at the beginning of the experiment is in order to calculate the mass of oxygen in magnesium oxide. Therefore, we could calculate the empirical formula of magnesium oxide.

Step 3 : Why do we need a crucible with lid for this experiment ?
We need a crucible with lid apparatus is to prevent strong light produced by magnesium which can let us blind.
Secondly, if magnesium is directly burn with fire, explosion will occur, which is really dangerous.
Thirdly, as we want to collect the most accurate result, we have to use the lid in order to avoid any substance lost in surrounding such as the white smoke or white powder substance.

Step 7: Why do we have the lift the lid frequently?
The purpose of lifting the lid is to let air goes in, and at the same time, is to avoid the escape of white smoke formed as it’s the magnesium oxide, which will led to inaccurate result because losing substance.

Is it possible to find any other empirical formula by using the same experiment ?

Yes, it is possible to find any other empirical formula by this.
For example : Silver oxide
4Ag + O2 ——; 2Ag2O

Reference
https://chemdemos.uoregon.edu/demos/Decomposition-of-Silver-Oxide

uncertainty of measurement
Compare to true value
Suggest why is not the same as true result
Do I have to write the gram in the table (right)
Accuracy vs precision
Type of error : random (unavoidable) and systematic

Ratio 2 : 1 : 2
2Mg(s) + O2 ——; 2MgO(s)

Ratio 4 : 1 : 2
4Na + O2 ——; 2NaO

Actual yield = come from our experiment
Theoretical yield = pencil and paper

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