Diagram:
Before I start any titration, I must first dilute the hydrochloric acid to a suitable level
To calculate the volume of hydrochloric acid I will add to the 250cm3 graduated flask, I must first calculate the molarity of the sodium carbonate.
For sodium carbonate (Na2CO3):
Moles = Mass
RMM
= 4.0
106
= 0.0377 mol dm-3 (3s.f.)
= 0.4 (1s.f.)
From the formula equation I can see that 1 mole of sodium carbonate reacts with 2 moles of hydrochloric acid, therefore the molarity of the acid will be double that of the sodium carbonate.
Concentration of hydrochloric acid = 0.04 x 2 = 0.08 mol dm-3
To make up a solution of 250cm3 of 0.08 mol dm-3 hydrochloric acid using 1.00 mol dm-3 hydrochloric acid I must find out how much of the acid, and therefore how much distilled water, I should add to the graduated flask for a 250cm3 solution.
Volume needed using = volume x concentration = V x C
1.00 mol dm-3 moles 1000 1000
= 250 x 0.08 1000
= 20cm3
According to the above equation, I will need to add 20cm3 of hydrochloric acid to a graduated flask that should also contain 230cm3 of distilled water to make up the rest of the 250cm3 solution of the acid. I should use a burette to add an accurate volume of hydrochloric acid to the graduated flask, and when filling up the flask a teat pipette should be used when making up the final few cm3 of the solution.
Risk analysis:
There is little risk involved in this experiment as all the solutions used are very dilute and are unlikely to harm anyone if spilled. However, for safety measures, a lab-coat and goggles should be worn and extra care should be taken when using the glassware.
Error Analysis:
The following table shows the error that can occur when reading off the measuring device. I must take these into consideration for my experiment as it could affect my results.
The error taken for the burette is as such because I am taking a reading from it twice (initial and final reading), but I am reading to the nearest 0.05cm3 to give a greater degree of accuracy. I will also need to take into account the degree of accuracy the titration was, but first I will have to see the volume of hydrochloric acid that will be used in the titration.
To ensure accuracy in this experiment I must:
- Wash all equipment with distilled water and then whatever may be put in it. i.e. wash the burette out with a bit of acid.
- Make sure there are no air bubbles in jet in the burette
- Make sure the jet is below the conical flask
- Make sure that there is no funnel in the burette when titrating
- Read from the bottom of the meniscus
-
Record all readings to the nearest 0.05cm3
- Make sure there are no air bubbles in the pipette
- Release solution and touch pipette onto surface
- Swirl the conical flask constantly
- Use single drops toward the end of the titration
- Record at least two concordant results after one rough titration
Results:
In the table below, I have tabulated my results for the titrations that I completed. The “Initial reading” and “Final reading” are indicating the level of hydrochloric acid used in the burette.
The average of this titration shall be taken from the most similar results. In this case they are: 16.70, 16.65 and 16.60.
Average titration = 16.70 + 16.65 + 16.60
3
= 16.65cm3
From the average titration I can work out the error percentage of the titration. This works out at 0.60%.
From the average titration volume I will be able to calculate the percentage purity of the sodium carbonate solution.
Moles HCl = Average HCl used x concentration HCl
1000
= 16.65 x 0.08
1000
= 0.00132
Moles Na2CO3 = moles HCl
2
= 0.00066
Moles Na2CO3 = Volume x Concentration
1000
Concentration Na2CO3= Moles Na2CO3 x1000
Volume
= 0.00066 x 1000
250
Percentage purity of = Concentration Na2CO3 x RFM
Na2CO3
= 0.0264x 106
= 0.27984
Moles HCl = 16.65x 0.08
1000
∴ Moles Na2CO3 = 16.65 x 0.08
in 25cm3 1000 x 2
∴ Moles in 250 cm3 = 16.65 x 0.08 x 10
1000 x 2
∴ Mass Na2CO3 = 16.65 x 0.08 x 10 x 106
in 250 cm3 1000 x 2
= 0.70596
∴Percentage purity = 70.596%
of Na2CO3
I now need to take into account the errors that could have been made.
Overall percentage = 0.50 + 0.24 +0.08 + 0.16 + 0.60 = 1.58%
error
So there is a possible error in my percentage purity of 1.58%,
Possible percentage = 70.596 ± 1.58 x 100
purity of Na2CO3 70.596
with error
= 73.2% to 68.4%
Therefore the percentage purity of the sodium carbonate solution could range from the upper bound of 73.2%, to the lower bound of 68.4%.
