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Mass of carbonate: This is the factor that I have chosen to vary. It will provide a suitable number of results for the experiment. The carbonate used will be calcium carbonate, and the acid will be 2 molar hydrochloric acid.
After choosing the factor that I will vary, I must take into account that all other key factors must stay constant. This is important, because then only do we know that it is the variable chosen that is affecting the volume of CO2 produced.
Preliminary Work
Preliminary work is done to find the ideal factor to vary and to find the appropriate levels to keep the other constants at. I will also hopefully find a suitable range of masses of carbonate to use.
Preliminary Apparatus
Plastic bowl
Measuring cylinder (100cm3)
Measuring cylinder (25 cm3)
Delivery tube
Buckner flask
Rubber bung
Calcium carbonate
Hydrochloric acid
Retort stand
Boss and clamp
Diagram
Preliminary Method
- A plastic bowl was half to three-quarters filled with distilled water.
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40 cm3 of hydrochloric acid was measured and poured into a conical flask.
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Once the rest of the apparatus had been set up as shown in the diagram above (with the conical flask held up by a boss, clamp and retort stand), a 0.1g mass of carbonate was added to the 40cm3 volume of acid in the conical flask.
- The rubber bung was replaced as quickly as possible (so that carbon dioxide didn’t escape through the top of the flask).
- Results of the volume of carbon dioxide were read from the measuring cylinder.
- The experiment was repeated using the volumes and masses shown in the preliminary results table.
Preliminary Results
Preliminary Conclusion
Firstly, I tried reacting a volume of 40cm3 of HCl with a mass of 1g of CaCO3. This reacted too quickly, so I decreased the volume of HCl. This produced a large volume of CO2, meaning that less had escaped because the speed of the reaction was reduced. I decided that the mass of CO2 was sufficient enough; therefore the volume of acid that I will use will be 25cm3.
I still needed to find the range of mass of carbonate I would use, so I tested low masses such as 0.10g and 0.15g. Then, I tested high masses such as 0.60g and 0.55g. From the preliminary results, I have decided that the range of masses of carbonate that will be used in the main experiment will be 0.15g – 0.55g.
The mass of carbonate will be in 0.05g intervals:
0.15, 0.20, 0.25, 0.30, 0.35, 0.40, 0.45, 0.50, 0.55
Accuracy
The measuring cylinder is an accurate enough means of measuring the volume of acid. It can be read to the nearest 0.1cm3, which is good enough at this stage.
It might be difficult to keep the volume of acid used exactly constant, but it will be near enough to constant.
The mass of carbonate may also not be exact, for two reasons. The scales used to weigh the mass might not be accurate enough, and when the carbonate is poured into the conical flask, some of it sticks to the plastic boat.
When the carbonate is added to the acid, there is a time period between the carbonate leaving the plastic boat, and the bung being put onto the conical flask. This time period may not be long, but it is enough for some of the CO2 to be lost. This reduces accuracy, but it would be difficult to change this problem, so it must be noted as a fault in the experiment.
Safety
There are corrosive and irritant substances such as acids being used; therefore I must be careful when pouring acids. Safety goggles must be worn at all times, and can only be removed when all apparatus has been put away, because there is still chance of a spillage, where acid could get into someone’s eyes and blind them.
Fair Testing
To keep this experiment fair, it needs to be accurate, reliable and precision is required when gathering data.
To increase reliability, the experiment will be repeated, so that two or three sets of results will be recorded. This depends upon how long it takes per set of results.
Precise data must be gathered. This means that as many decimal places or significant figures as possible must be used. The mass of carbonate must be measured to as close to 5cm intervals as possible, the volume of acid must be as close to 25cm3 as possible, and the reading of the volume of CO2 off the measuring cylinder must be done carefully, and precisely.
Hypothesis
From evidence obtained in the preliminary work, I predict that the volume of carbon dioxide will increase as the mass of carbonate does. (As the mass of CaCO3 increases, the volume of CO2 increases).
I predict that the graph of results will look similar to this:
Volume of CO2
Mass of CaCO3
I have decided to do some calculated predictions. These will be useful to check my main results by.
The ratio of CO2:CaCO3 is 1:1
This means that the volume of CO2 produced is directly proportional to the mass of CaCO3. (I.e. if the mass of CaCO3 is doubled, then the volume of CO2 produced will double).
At 25˚C and at atmospheric pressure, the volume of 1 mole of ANY GAS is 24dm3
24dm3 = 24000cm3
1 mole of CO2 occupies 24000cm3
RFM of CaCO3 = 40.0 + 12.0 + (16.0 x 3) = 100
Volume of CO2 produced = Area that 1 mol of CO2 occupies
Molecular Mass of CaCO3 x Mass of CaCO3
Therefore,
1g of CaCO3 will produce 24000
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x 1 = 240cm3
Prediction Table
After making these calculated predictions, I can see how inaccurate the preliminaries were. I decided that some of the carbon dioxide must be escaping between the time the carbonate is poured in, and the bung is put on. I must figure out a new method which will ensure that no carbon dioxide escapes, and all of it is collected.
I must also change the range of masses of carbonate, because as we can see from the prediction table, 0.45g, 0.50g and 0.55g all give volumes of CO2 of more than 100cm3. This is bad, because the measuring cylinder scale only goes up to 100cm3. Therefore, the new range that I shall use is…
0.10, 0.15, 0.20, 0.25, 0.30, 0.35, 0.40
This should give enough results to ensure that the experiment is a success.
After researching more apparatus that could be used to ensure that no carbon dioxide escapes, I have decided to change the method that will be used to collect the main results.
Also, after doing a calculation of excess I know that a volume of acid higher than 4.2cm3 is needed. I used 25cm3 in the preliminaries, so this should work well.
Revised Method
- The apparatus will be set up as before, but the carbonate will not be poured into the acid.
- Instead, the carbonate will be put into an ignition tube.
- This will then be put into the conical flask, and the bung will be put on.
- To start the reaction, the conical flask will be shaken, so that all of the carbonate reacts with the acid.
- The volume of carbon dioxide produced will be read off the scale of the measuring cylinder.
- The experiment will be repeated to ensure reliable results.
One more preliminary result was recorded to prove that this revised method worked. I reacted 0.40g of CaCO3 with 25cm3 of acid and 88cm3 of CO2 was produced. This result is much more accurate than before and closer to the predicted 96cm3.
Obtaining Evidence
Method
- A Buckner flask was held up using a retort stand, boss and clamp. A delivery tube was attached.
- A plastic bowl was filled with water, and a measuring cylinder was also filled with water. This was tipped over, so no water was lost into the bowl and the delivery tube was placed so that it was inside the measuring cylinder.
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25cm3 of HCl was the poured into the Buckner flask (As close to as possible, because we know that this doesn’t matter as it is in excess from the excess equation). And 0.1g of CaCO3 (in an ignition tube) was dropped into the flask, and the rubber bung was replaced onto the flask.
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To start the reaction, the Buckner flask was shaken, so that all of the CaCO3 reacted with the HCl.
- The volume of carbon dioxide produced was read off the scale of the measuring cylinder.
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The experiment was repeated using different masses of CaCO3.
- The whole thing was then repeated two more times, so that three sets of data had been collected. This was to endure reliable results.
Results
Analysing and Considering Evidence
In the Hypothesis, I said
“I predict that the volume of carbon dioxide will increase as the mass of carbonate does. (As the mass of CaCO3 increases, the volume of CO2 increases)”
This prediction was correct, as can be seen in the results table. The volume of CO2 produced is directly proportional to the mass of CaCO3 used. This is because the carbon and oxygen that go into the production of CO2 are both found in the CaCO3. This means that the maximum volume of CO2 that can be produced increases when the mass of CaCO3 does.
In the Hypothesis, I also showed what the graph of results should look like. This was also accurate. Below are three graphs. The upper one is my predicted graph of results. The lower left one is the graph of results based on my earlier prediction table. The lower right one is the graph of the actual results.
Volume of CO2
Mass of CaCO3
Although the general trend of my actual results is much the same as that of the predicted one, it is clear to see that there are faults. In the cases where 0.3g and 0.35g of CaCO3 were used, the volumes of CO2 given were higher than they should be according to my predictions. This means that the mass of CaCO3 used was probably not 0.3g or 0.35g in each case, but more. This is because the maximum amount of CO2 produced with 0.3g of CaCO3 is 72cm3, however, the average volume collected was 73.7cm3. This mistake was most probably made because when measuring the mass of CaCO3, not enough care was taken. However, it could also be because the scales used to measure the mass only gave the mass to a precision of two decimal places. Therefore, the mass of 0.3g for example could be between 0.295000…g and 0.304999…g. This is a difference of 0.00999…g, which could be a lot of CO2 produced.
The evidence obtained could also have been affected by the loss of gas. Although the method was improved to reduce the loss of CO2, some is still lost as can be seen in the results. Also, mistakes could have been made reading off the side of the measuring cylinder.
Fortunately in this experiment, no anomalous data was collected. Anomalous data is data that does not fit the trend or pattern of the other data collected. It does not show the accuracy, merely the reliability.
Evaluation
In the preliminary work, the procedure used to obtain evidence wasn’t very good as I noticed a lot of the carbon dioxide was lost through the top of the Buckner flask. The revised method, with the use of an ignition tube was much better and gave much more accurate results. However, some carbon dioxide was still lost, because when the ignition tube falls in the flask, it lands on its side, allowing the carbonate to react with the acid immediately. This wouldn’t be as much of a problem if the volume of carbon dioxide lost each time was the same. Unfortunately, this is not the case, and the volume lost isn’t constant, so the results shown in the graph do not show a straight line.
If the volume of carbon dioxide lost each time was the same, the line would be straight, but it would be below the correct one, but as the volume lost was different each time, the line is not straight.
The procedure would be improved by finding a way that the acid can be stopped from reacting with the carbonate until after the bung is put on the Buckner flask. The method using ignition tubes instead of simply pouring the carbonate into the flask did this, but as we can see, carbon dioxide was still lost. More ways to improve the procedure need to be researched.
The evidence obtained was reliable in that no anomalies were spotted. However, reliability isn’t the same as accuracy, and there were parts of the results where it was accurate and parts where it was less so, but throughout it was reliable. The fact that the results weren’t so accurate, but were reliable shows that the same problem was occurring at the same level in each repetition of the experiment.
From the evidence obtained, we can see that the range of masses of carbonate could be extended to gather more data. The minimum volume used could be 0.05g (in keeping with the 0.05g gap between masses used). The maximum mass used could not, be extended unless a larger measuring cylinder is used.
One reason why the volume of carbon dioxide produced was more than it should have been is that there were already a few bubbles in the measuring cylinder from where it was tipped over. This meant that these air bubbles would have been added to the volume of carbon dioxide produced, making the results slightly inaccurate. Taking further care when inverting the measuring cylinder could prevent this problem.
