Determining the concentration of acid in a given solution
I have been given a sample of sulfuric (VI) acid solution with a concentration between 0.05 and 0.15 mol dm-³. I am going to find out the accurate concentration of the sulfuric acid. To find out the concentration of the acid I will react it with a known volume and concentration of a base and see how much base was needed to neutralise the acid.
The acid is a strong acid which means that I know all the H+ ions have been disassociated and are in the solution. The H+ ions will react with the OH- ions in the alkali which will neutralise the solution.
I am provided with solid, hydrated sodium carbonate with the formula Na2CO3·10H2O.1 This is a readily available base and I can dilute it down to achieve the concentration I want to react with the acid.
The formula of the reaction that will take place is
H2SO4 (aq) + Na2CO3 (aq) → Na2SO4 (aq) + CO2 (g) + H2O (l)
So 1 mole of H2SO4 reacts with 1 mole of Na2CO3.
A titration will give me the most reliable and accurate results with the available equipment.
To do my titration I will need:
A Burette 7
I will need a burette to add the sodium carbonate to the sulfuric acid solution. The burettes provide me with very accurate results of volume of solution added.
The class set of burettes measure 50cm3. I want to do a titration of around 25cm3 - 35cm3 each time. This is because the larger the amount of solution I titrate, the smaller the percentage error. I don’t want to plan for each titration to exceed 35cm3 as if each titre is slightly larger then I may run off the scale of the burette. If I had to refill my burette during a titre, then the percentage error would double as I would have to read the burette at the beginning and the end of the first amount of solution and again at the beginning and end of the second solution.
I can read the burette to the nearest 0.05cm3 so the precision error is ±0.025cm3 for each reading. I will have to read the burette at the beginning and end of the titre so the precision error for each titre is ±0.05cm3.
If the titre is 30cm3 the error is (0.05/30.00) x 100 = 0.16% which is acceptably low.
A pipette 7
I will need a very accurate, glass pipette which I can use to transfer a specific known volume of sulfuric acid added to the conical flask to be reacted.
There are class sets of pipettes of 10cm3 and 25cm3. I have a large amount of sulfuric acid, so I am not limited to using small volumes of sulfuric acid each time.
A pipette measuring 10cm3 is accurate to ±0.04cm3, so has a percentage error of 0.4%. A pipette measuring 25cm3 has a precision uncertainty of ±0.06cm3, yet because of the larger volume it measure, it has a percentage error of 0.24%.
I am going to use a pipette which measures 25cm3 of solution as I am not limited on the amount of solution I can use and it will provide me with the least percentage error and therefore mean my results will be more accurate.
A volumetric flask 7
I will use a volumetric flask to create the correct concentration of sodium carbonate solution. I will add the mass of solid sodium carbonate I need and then fill the volumetric flask up with distilled water, so that there is a know concentration of the solution.
The error on a 250cm3 volumetric flask is 0.2cm3 if read correctly.
This means that the percentage error is 0.08%. This is a very low percentage error for the volumetric flask and will have very little effect on the overall concentration of the sodium carbonate solution.
I know that the concentration of sulfuric acid is between 0.05 and 0.15 mol dm-3. To do some estimation calculations I will presume that my acid is 0.10 mol dm-3. I will use 25cm3 of the acid in a conical flask as this is how much I will be able to add accurately using a 25cm3 pipette.
This means that there will be 0.10 moles H2SO4 in 1000cm3
(0.10/1000) in 1cm3
((0.10/1000) x 25) = 2.5x10-3 moles in 25cm3
In the reaction between the acid and base, the disassociated H+ ions and OH- will react to neutralise the solution. I am going to use a similar concentration of sodium carbonate and sulfuric acid so that when the reaction reaches the endpoint, all the H+ ions have reacted and the OH- ions are not in excess. I will therefore use a concentration of approximately 0.10 mol dm-3 as this is the middle value of the possible concentration of sulfuric acid.
1 mole of H2SO4 will react with 1 mole of Na2CO3
Using an approximate titre of 30cm3 I will need
As these calculations were all based on approximate values I will round 0.083 up to 0.1 mol dm-3, as 0.1 is an easier concentration to make up and will ensure that there are enough OH- ions to react with the H+ ions.
I am using titres of approximately 30cm3. I will need some sodium carbonate solution to rinse the burette, solution to do a rough titration, and I will want about 5 titres worth to get 3 accurate titres so I will need (7x30)= approximately 210cm3 of sodium carbonate solution.
I will use a volumetric flask to measure up the amount of sodium hydroxide solution needed. There are volumetric flasks of 100cm3 and 250cm3. I will use the 250cm3 flask as this will provide me with enough solution to do all my titres and will minimise the percentage error as the volume it carries is larger.
Measuring the sodium carbonate solution 1 2
I need to make 250cm3 of a sodium hydroxide solution with the concentration of 0.1 mol dm-3. I will use solid hydrated sodium carbonate with the formula Na2CO3·10H2O.1