To clear this up we use John Dalton’s theory, which supports the law of constant composition. His theory basically shows that the atoms of different elements could be distinguished by differences in their weights. Here is his theory in short:
- All matter is composed of atoms
- Atoms cannot be made or destroyed
- All atoms of the same element are identical
- Different elements have different types of atoms
- Chemical reactions occur when atoms are rearranged
- Compounds are formed from atoms of the constituent elements.
Dalton atomic theory also showed that an element is pure because all the atoms are identical and also that compounds consisted of different elements combined together.
To try and see if my hypotheses are correct I did a preliminary experiment.
Preliminary Expt.
Method
Measure the weight of the crucible and the lid with and without the magnesium in it before and after it has been oxidised.
Safety
- Use tongs to hold the crucible, as it will be very hot coming from the Bunsen burner
- Wear goggles at all times during the experiment
- Always stand up during the experiment
- Weigh the crucible after it has cooled down after the experiment
Fair test
Always use the same range of time during this experiment as it would be unfair if the range of time varied.
Results
I weighed all the masses and these are the results I obtained:
Mass of crucible + lid =22.83g
Mass of crucible+ lid + Mg= 23.04g
Mass of crucible + lid + product when cool= 23.18g
Mass of Mg used=0.21g
Mass of MgO formed=0.35g
Mass of combined oxide=0.14g
Calculations
As you can see above my results has the perfect composition with one another. So hopefully my final experiment will have similar success.
Now I shall proceed with my final experiment
Apparatus
Gauze
Tripod
Safety Map
Tongs
Bunsen burner
Crucible (of course with the lid)
Magnesium
Stopwatch
Scales
Goggles
Burning mat
Diagram
Variables
Independent variable
The mass of the magnesium burnt
Dependent variable
The mass of oxygen combining with magnesium
Controlled Variables
Same type of magnesium strip
The period of time during this experiment
The same amount of time to be cooled down
The same temperature, this can be done by using the same Bunsen burner all the way throughout this experiment.
Method
Ø set up the apparatus as shown above
Ø then weigh the crucible and the lid, without the magnesium inside it and record that weight.
Ø weigh the crucible with the magnesium strip inside of it, whatever length will do, however it is easier to do it systematically either largest to smallest or smallest to largest. Record the weight again.
Ø Put the crucible onto the gauze (shown in the diagram) and switch on the Bunsen burner.
Ø Start the stopwatch and proceed with the experiment. We will be using 10 minutes as the range of time during this experiment. After every minute check the magnesium by lifting the lid using the tongs. Only do it for a few seconds, as we do not want the oxygen to escape from the crucible.
Ø After the ten minutes turn off the Bunsen burner and check if the magnesium has been burnt. If there is some still left burn it for an additional 5 minutes and then reweigh the crucible with the oxidised magnesium in it. Remember to wait a few minutes for the crucible to cool down as it is very hot after the experiment and we do not want to damage the scales. Also when waiting for it to cool down place it on a burning mat so the table does not get damaged.
Ø repeat this process with all the lengths of magnesium and record the weights in the table below:
To calculate the mass of magnesium used and the mass of oxygen combined I use this:
Mass of magnesium used
Mass of crucible+ lid+ magnesium – mass of crucible + lid
Mass of oxygen combined
Mass of crucible + lid + product cold – mass of crucible + lid + magnesium
Safety
- A burning mat must be used as we do not want to cause a fire or burn any of the surroundings during this experiment
- Wear goggles at all times as the reaction between magnesium and oxygen is violent and the light and particles produced from the reaction can damage your eyes.
- Do not look directly into the crucible when the reaction is happening, as the light produced from the reaction is very intense.
- Wait for the crucible to cool down after the experiment, as we do not want to damage the scales, because they are very sensitive.
- Tongs are needed so you do not touch the hot crucible.
- Always stand during this experiment, as there is a possibility of the experiment toppling over, so when it does happen you can quickly jump out of the way.
- Do not play about with the magnesium as the edges are very sharp and can cut you if you are being foolish with it.
- Tuck in any loose hair or ties as they may catch fire during this experiment.
Fair test
- Use the same Bunsen burner, because if you change the controlled variables, in this case temperature, then the experiment will definitely unfair
- Make sure that every experiment has the same amount of time to be burnt and to be cooled down.
- Use the same type of magnesium.
Results
From the results I have obtained I have produced a table of results and they are as followed:
Also I have the results obtained from the whole class. Some have been left out, as they are either striking anomalies or were not done fully. This is helpful because now I have more results and I can compare the class’ results with my mine so I can produce a better conclusion.
The masses are measured in grams.
Graphs
The graphs below show the relationship between the mass of magnesium and mass of oxygen. The graph below is of the results the whole class obtained.
The next graph is of my results
Calculations
To proceed with the calculations we will firstly know how to use the empirical formula.
The empirical formula shows the simplest ratio in which atoms combine, if you know the masses of the elements then you can work out the formula:
- Find the masses that combine (in grams) by experiment
- Change grams to moles of atoms
- This tells you the ratio in which atoms combine
- Then write the formula down
Below are the empirical formulas for the experiment I carried out.
5cm
7.5cm
10.0cm
12.5cm
15.0cm
Conclusion
From the results I have obtained you can see that there is some sort of correlation between my hypotheses and results. Although from the calculations I made using the empirical formula do not show a perfect relationship (in this case composition) which would ideally be 1:1, my results are not far off it. For example take the results I obtained from 5cm of magnesium, using the empirical formula I was able to get a ratio of 1.11:1, which is very close to the ideal ratio of this experiment. This shows that I did do the experiment well in some areas.
The results I received from the class are very varied but in some places do show some good relationships. For example in the places I have circled show some correlation as they have similar weights to each other meaning that they would have similar ratios to each other.
Some of my predictions were correct as some ratios were near perfect showing that the compositions did not or slightly change.
Evaluation
Although I personally found my results to be reasonable, it is quite clear that there are spaces for improvement, as my results did not totally correlate with my hypotheses. Also the class results are very random and varied showing that they made severe errors and mistakes during this experiment. Thus showing several anomalies on the graph of results.
The errors could have occurred because of not doing the experiment properly. For example I for one lifted the crucible lid for too long at one point, this could have meant that some magnesium vapour would have escaped or the magnesium would have reacted with the air which also contains large amounts of nitrogen, thus causing magnesium nitride and possibly hindering my results during my experiment. Also what I had noticed around the class is that not everybody burned all the magnesium in the crucible; this meant that their results would have been incorrect because for this experiment to work all the magnesium in the crucible should have been burnt. However, this was always certain to happen, as some people in the class do not listen to instructions. Another problem could have been the scales; sometimes they could have not been reset after someone else has used them meaning that some weights could have been recorded incorrectly, however this could have been marginal because it is usually out of place by around 0.01 grams.
All these factors can be resolved but only in prestige laboratory conditions.
If I did this experiment again throughout the experiment I will only lift the crucible lid a little bit so I can see what is going on. Also I would take down more results, I would change the ranges of length of magnesium, instead of gathering results form the class because their results can be seen as unreliable as they may not have followed the process of the experiment like I did. Obviously 20 results are better than 5 as they produce a more reliable conclusion.
Also I would change the way I proved my conclusion, instead of just doing the empirical formula for magnesium oxide, I can find the percentage composition of the compound and compare them to create a better conclusion.
Christopher Baziwe 11OL
