Conc. of H2SO4 (C) = 0.05 mol/dm-3
Volume of H2SO4 (V) = 25cm3 = 0.025 dm3
Moles of H2SO4 (n) = CV = 0. 05 x 0.025 = 0.00125 mol
0.025dm3 of Na2CO3 solution contains 0.0025 moles of H2SO4
Concentration of Na2CO3 = = 0.00125 = 0.05 mol/dm-3
0.025
n = mass
Relative molecular mass
Number of moles (n) of Na2CO3 = 0.00125mols
Relative molecular mass of Na2CO3 = (23 x 2) + 12 + (16 x 3) = 106g
∴ Mass of Na2CO3 = n x relative molecular mass
= 0.00125 x 106 = 0.1325g
If 25cm3 contains 0.1325g of Na2CO3,
250cm3 will contain 0.1325 x 10 = 1.325g
≈ 1.33g
Assuming the concentration of H2SO4 to be 0.15 moldm-3, the concentration of Na2CO3 can be calculated as follows:
Conc. of Na2CO3 (C) = 0.15 mol/dm-3
Volume of Na2CO3 (V) = 25cm3 = 0.025 dm3
Moles of Na2CO3 (n) = CV = 0.15 x 0.025 = 0.00375 mol
0.025dm3 of Na2CO3 solution will contain 0.00375mols of H2SO4
Concentration of Na2CO3 = = 0.00375 = 0.15 mol/dm-3
0.025
n = mass
Relative molecular mass
Number of moles (n) of Na2CO3 = 0.00375 mol
Relative molecular mass of Na2CO3 = (23 x 2) + 12 + (16 x 3) = 106g
∴ Mass of Na2CO3 = n x relative molecular mass
= 0.00375 x 106 = 0.3975g
If 25cm3 contains 0.3975g of Na2CO3,
250cm3 will contain 0.3975 x 10 = 3.975g
≈ 3.98g
From these calculations, it is clear that I must weigh accurately a sample of Na2CO3 between 1.33 and 3.98g. The average mass of these 2 figures is 2.65g, and I shall use this amount to make a solution of Na2CO3 with the concentration of about 1.0 mol/dm-3.
RESULTS AND CALCULATIONS:
I will record the results from the experiment in the table below:
TITRATION OF A SOLUTION OF SODIUM CARBONATE WITH SULPHURIC ACID OF CONCENTRATION BETWEEN 0.05 AND 0.15mol/dm-3.
After making the solution of sodium carbonate of known concentration, I will titrate it with sulphuric acid.
Na2CO3(s) + H2SO4 (aq) Na2SO4 (s) + H2O (l) + CO2 (g)
Sodium + sulphuric Sodium + Water + Carbon dioxide
Carbonate acid Sulphate
(Salt)
I have to decide which indicator to use in helping me to know the stiochiometric point or equivalent point of the reaction, which will signify the end of the reaction.
This graph shows the equivalence point of a strong acid/weak base titration; the band of colour shows the colour the methyl orange will turn. Because sulphuric acid is a strong base and sodium carbonate is a weak alkali this shows that most suitable indicator is methyl orange which will turn from red to yellow once the equivalence point has been reached.
APPARATUS:
- Safety glasses
- Small filter funnel
-
50cm3 Burette
- Boss Head, Clamp & clamp stand
-
2 x 100cm3 beakers
-
Sulphuric acid, of concentration between 0.05 and 0.15mol/dm-3
-
25cm3 Pipette
- Pipette filler
- Sodium carbonate of known concentration
-
4 conical flasks, 250cm3
- Methyl orange indicator
- White tile
- Wash bottle of distilled water
Diagram
Plan
- Using the funnel, I will rinse the burette with sulphuric acid (rinsing and filling the tip) and fill it with same solution so that the bottom of the meniscus is level with the 0 mark.
-
I will rinse the pipette with some solution of sodium carbonate and carefully transfer 25.0cm3 of the solution into a clean 250cm3 conical flask.
- I will add 2-3 drops of methyl orange indicator solution
-
I will run the sulphuric acid solution from the burette into the flask, with swirling until the solution just turns yellow. This will be used as a trial run as I will probably overshoot the equivalence point. I will record the final burette reading to the nearest 0.05cm3.
- I will then refill the burette with sulphuric acid to the 0 mark.
- Using the pipette, I will transfer 25.0cm of the sodium carbonate into another clean conical flask, adding 2-3 drops of the methyl orange indicator solution.
- Carefully, I will titrate this solution to the end point, adding the acid drop by drop when the colour is about to change.
- I will repeat steps 5 - 7 at least twice more and keep flasks for comparison, my equivalence point should get more accurate with each repeat.
Results
CALCULATIONS:
Using the data obtained from the titration, I will calculate the concentration of the sulphuric acid. Knowing most accurate titre, volume and concentration of the sodium carbonate, I will calculate the number of moles of sodium hydroxide and use my previous calculations to find the concentration of the sulphuric acid:
First I need to work out the moles of both solutions.
Moles Na2CO3 = Conc X Vol
= 0.1mol/dm-3 X 0.025
= 0.0025
The concentration & the volume of Na2CO3 are already known.
Moles H2SO4 = 0.025 Volume = 0.02025
As the ratio of the acid & base is 1:1, the number of moles will be the same. The volume is my average titration in dm3.
Next, I will work out the concentration of the H2SO4:
Concentration = Moles _ = 0.025
Volume 0.02025
= 0.1235
≈ 0.124 mol/dm3
This number is the concentration of sulphuric acid as I have worked out.
I will now use this figure to work back through my calculations to check the concentration of the Na2CO3. If this figure is 0.1, then this shows my calculations to be accurate:
2.65g dissolved in 250cm3 H2O = 0.25 dm3
Conc = Moles Moles = Mass = 2.65 = 0.025 mol
Vol Mr 106
Conc = 0.025
0.25
Conc = 0.1 mol/dm3
This proves that my calculations are accurate.
RISK ASSESSMENT
Sources used
- Salter’s Advanced chemistry, Chemical Ideas
- Salter’s Advanced Chemistry, activities booklet
- Hazard Cards
- http://www.chemguide.co.uk/physical/acidbaseeqia/indicators.html (for info about indicators)
- planning assessment information sheet]
Evaluation
The equipments I choose were such that accuracy, precision and reliability are ensured.
Volumetric pipette (25cm3):
This was used because of its high degree of accuracy in quantitative analysis. It has an error of 0.06cm3 if used correctly (i.e. if it is allowed to drain and retain the last drop).
The percentage error = error x 100 = 0.06 x 100 =0.24%
Reading 25
This is a relatively small percentage and ensures that the volume of solution measured is accurate to the highest degree.
Volumetric flask (250cm3):
This is used to make standard solution of a particular volume. If a 250cm3 volumetric flask is filled correctly i.e. the bottom of the meniscus rests on the calibration line, the error is 0.2cm3.
The percentage error = error x 100 = 0.2 x 100 =0.08%
Reading 250
The percentage error is low and therefore the flask measures the volume to a high degree of accuracy.
Burette:
One drop from a burette has a volume of approximately 0.05cm3. All the burette readings should include 2 decimal places in which the second figure is either 0 or 5. An error of one drop in a volume of 25.00cm3 gives a percentage error:
The percentage error = error x 100 = 0.05 x 100 =0.2%
Reading 25
This is also low, meaning that the equipment has high accuracy.
Beakers:
The beakers are useful for measuring broad volumes of a solution. This is not very accurate but can give an approximate volume. I used it to measure the volume of water because the volume of water needed was approximate.
Conical flask:
These are good for titrations as they have a narrow neck (enough to fit in the burette tip, without breaking it when swirling), but broad base. This is particularly useful as the narrow neck prevents spillage when swirling and the broad base makes the solution and end-point easy to see.
Weighing balance:
A two place weighing balance has an error to 0.01g. Using it to measure a mass of 5g will have percentage error:
The percentage error = error x 100 = 0.01 x 100 _ =0.2%
Reading 5
This is low and has high degree of accuracy.
Apart from the errors and uncertainties related to the precision of the equipments used, procedural error may arise from practical techniques. This would include:
- Not mixing the solution in the volumetric flask thoroughly would make the solution have uneven concentration through out. Some areas would be more concentrated than others. A lower concentration would use less volume from the burette and this would make my result lower. A higher concentration would use more burette solution and would make my result higher.
- Not washing the burette and pipette with the solutions they are to contain before titrating can be a source of error as other substances may be present in the equipment. This can affect the concentrations of the solution, hence the volume of the titre used. The substances that will increase the value of the titre will make my result higher and the substances that will reduce the value of the titre will make my result lower.
- The conical flask needs to be thoroughly rinsed with distilled water in between titrations. This will remove any solution from the previous titration and not affect the concentration of the solution in which it is to contain, as the concentration of the solution is already known.
The mass of sodium carbonate (between 1.33-3.98g) I used in making the standard solution of sodium carbonate was ideal because it was calculated theoretically taking into consideration the given concentration of sulphuric acid (between 0.05-0.15mol/dm3).
The volume of sodium carbonate used in the titration (25cm3) was appropriate because using a 25cm3 pipette gives more accurate and reliable result than using a pipette with a greater volume capacity. Using a pipette of volume less than 25cm3 would not make available enough solution for the titration and I would be working with very small values that can cause errors. The 25cm3 pipette I used is not too small or too big, it is just reliable. Moreover, this is used in most standard titrations.
Making the solution of sodium carbonate to a volume of 250cm3 in a volumetric flask made available enough solution in case I overshoot pass the end-point and need 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.
01/05/2007 Deducing the quantity of acid in a solution Page