I will weigh a weighing boat on a balance correct to three decimal places, record the result, and then add approximately 2.650g of the sodium carbonate and record the result. I will tip the powder into a beaker, and then re – weigh the weighing boat. To find the exact amount of sodium carbonate, I will minus the last weighing from the second weighing. This is because a very small amount will remain in the weighing boat, stuck to the sides.
To the solid in the beaker I will add distilled water and stir them with a glass rod until they have all dissolved. When all of the solid has dissolved, I will get a funnel and a 250cm3 volumetric flask, and run the solution down the glass rod, into the funnel, and into the volumetric flask.
I will then rinse the beaker, rod and funnel, 5 times with more distilled water, to make sure that all the solution has been transferred. Then I will make up the rest of the solution by filling the volumetric flask with distilled water, until it is just underneath the calibration line. Then I will drop in the distilled water with a plastic pipette, until the bottom of the meniscus is on the line at eye level.
The flask will be stoppered and inverted 20 times to make sure the solution is mixed.
To prepare the burette
A clean dry burette will be taken and rinsed out, using a funnel, with about 10 cm3 of sulphuric acid. This is done by moving the acid up and down in the burette, while rotating it, so that the acid touches all of the burette. I will then let the acid out through the tap, so that it rinses that as well.
It will then be placed in a boss and clamp on a stand. I will bring the stand down off the bench on to a stool so the top of the burette is not so high up, and using a clean funnel I will fill the burette with sulphuric acid to a little above the 0.00 line. I will remove the funnel and place a beaker under the tap and let the jet fill with acid, making sure there are no air bubbles. When the bottom of the meniscus is on 0.00 I will shut the tap.
The burette will then be ready for the titration.
To prepare the standard solution – using the pipette.
I will us a 25cm3 pipette and filler to transfer 25cm3 of sodium carbonate solution into a 250cm3 conical flask.
Firstly I will rinse the pipette by sucking a little of the solution up and swirling it around so it touches all the sides. I will discard this solution, and then add my first 25cm3.
When the pipette has stopped draining I will touch the tip on the bottom of the flask twice.
Because sulphuric acid is a strong base and sodium carbonate is a weak alkali the most suitable indicator is methyl orange. I will add 5 drops of methyl orange indicator, and place the conical flask under the burette, on a white tile. The white tile will make the colour change from yellow to orange more apparent.
The titration
I will carry out a rough titration first, to get an idea of how much acid I will be using.
I will add the acid in 1cm3 portions, while swirling, until the end point is just passed, and the solution in the conical flask has turned orange. The result will be recorded, and the solution discarded. I will wash out the conical flask with distilled water, ready for the accurate titration.
I will then re-fill the burette up to 0.00 with more sulphuric acid, using the funnel.
I will add the acid to the alkali solution rapidly this time, until I am within 1 cm3 of the rough titre. From then I will add the acid drop by drop, swirling the conical flask in between each drop.
I will repeat this procedure until I get 3 concordant results within 0.1cm3 of each other.
The results will be presented in a chart showing starting point, end point, and titre. Then the mean titre will be calculated.
Sources:
- Experiment “ find the number of moles of water of crystallisation in washing soda”. This was a titration experiment, in which the standard solution was already made up.
- Experiment “ To find the solubility of calcium hydroxide in water by titration”
In this I had to make up the standard solution and titrate.
- Experiment EL 2.1 “ How much iron is in a sample of an iron compound?” This involved making up a standard solution and titrating.
- Chapter 1.5, Chemical Ideas – “Concentrations of solutions”.
- Chapter 4.2, Nuffield advanced chemistry student book – “The chemists’ toolkit: solution concentration”
ANALYSIS
To know exactly how much of the sodium carbonate I have used I did these calculations:
Mass of boat = 1.035g
Mass of boat + sodium carbonate = 3.688g
Mass of boat after emptying sodium carbonate = 1.038g
Therefore:
Mass of sodium carbonate used = 3.688 – 1.038
= 2.650g
This is the amount that I calculated I should have used, and it has happened to come out exactly right.
When the titration was done, these are the results I obtained:
Then I worked out the average titre:
(25.30 + 25.40 + 25.40) / 3 = 25.37cm-3
To find out the concentration of the acid, I need to know the exact concentration of the standard solution. Here is how I did it:
Concentration = No. of moles / volume
To find the number of moles this has to be done:
No. of moles = mass / RMM
= 2.650 / 106
=0.025 moles
So therefore:
Concentration of alkali = No. of moles / volume
= 0.025 / 0.250
= 0.100 mol dm-3
To find out the number of moles in the solution in the conical flask, the actual amount being titrated, which was 25 cm3, I have to divide the number of moles in the volumetric flask by 10, because 250 / 25 is 10.
No. of moles of standard solution used = 0.025 / 10
= 0.0025 moles
If I look at the equation, I can see that 1 mole of sulphuric acid reacts with 1 mole of sodium carbonate, so in my experiment, 0.0025 moles of acid will have reacted with the 0.0025 moles of sodium carbonate.
H2SO4 (aq) + Na2CO3 (aq) → Na2SO4 (aq) + CO2 + H2O
Now that I know the number of moles of acid used, and the volume of the average titre, I am in a position to work out the concentration of the acid.
Concentration of acid = No. of moles / volume
= 0.0025 / 25.367cm3
= 0.0025 / 0.0254 dm3
= 0.0984 mol dm-3
I have worked out the concentration of the sulphuric (VI) acid to be 0.0984 mol dm-3.
EVALUATION
During my experiment, lots of minor errors will have occurred. These are expected when doing an experiment where you have to use measuring apparatus, which are only designed to measure to a certain level of precision. The apparatus in this experiment, which fits that description are the balance, the burette, the volumetric flask, and the pipette.
The balance I used gave the mass to 3 decimal places. To calculate the % error, these calculations were done:
% Error of balance
Mass of boat = 0.0005 x 100 / 1.035
= 0.0483 %
Mass of boat + Na2CO3 = 0.0005 x 100 / 3.688
= 0.0136 %
Mass of boat after emptying = 0.0005 x 100 / 1.038
= 0.0481 %
% Error of burette
% Error of average titre = 0.025 x 100 / 25.37
= 0.0985 %
Because there is only one marking on the 25cm3 pipette, and the 250cm3 volumetric flask, the precision was looked up in ‘‘the individual investigation: notes for students’’.
% Error of pipette = 0.06 x 100 / 25
= 0.240 %
% Error of volumetric flask = 0.2 x 100 / 250
= 0.080 %
These percentage errors are very low. This is because I have used very accurate apparatus, as titrations have to be as precise and accurate as possible.
I think I have achieved very reliable results. No anomalous results were obtained. This is due to the accuracy of the instruments I have used and also due to attention to detail. Without this, the experiment would not have been as reliable.
Making the solution of sodium carbonate in a 250cm3 volumetric flask made sure there was enough solution available, in case I overshot the end-point and needed to repeat the titration. This also ensures that there is enough solution to repeat titrations so that different values of titre will be collected and the average of them taken. This ensures accuracy.
To make sure all the sodium carbonate went into the solution in the volumetric flask, I rinsed the glass rod, funnel and beaker 5 times. The water will have made sure that every last molecule of sodium carbonate weighed out, was in the solution.
Mixing the solution in the volumetric flask made sure that the solution had an even concentration throughout. If I did not invert it 20 times, some areas would be more
concentrated than others.
Washing the burette and pipette with the solutions they are to contain before titrating eliminated a lot of error, as other substances may have been present in the equipment. This can affect the concentrations of the solution, hence the volume of the titre used.
The conical flask needed to be rinsed with distilled water in between titrations. This removed any solution from the previous titration and so did not affect the concentration of the solution of which it is to contain, as the concentration of the solution is already known.
All reading of menisci were taken at eye level, and read from the bottom of the meniscus.
Because sulphuric acid is a strong base and sodium carbonate is a weak alkali the most suitable indicator was methyl orange. I decided to have the acid in the burette and the alkali in the conical flask because I feel that it is easier to see a colour change from yellow to orange than orange to yellow. But identifying the end point was still a problem, which may have led to inaccuracies in the results.
Using a white tile made it easier to tell when the colour change in the indicator had occurred,