To get a 0.05M succinic acid solution in 250cm3 will take how many grams of solid acid if ‘n=3’ and the molecular mass is 132g ?
132g in 1dm3 = 1M
13.2g in 1dm3 = 0.1M
6.6g in 1dm3 = 0.05M
1.65g in 250cm3 = 0.05M
Therefore if ‘n=3’ its going to take 1.65g of succinic acid in 250cm3 to make a 0.05M solution, which is what is needed for the titration.
Preparation of Acid Solution:
To make a 0.05M solution once we know how much solid acid to dissolve (1.65g) is quite simple.
- Transfer between 1.64 and 1.66g of succinic acid into a weighing bottle and weigh it to the nearest 0.01g (using an accurate digital balance).
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Put about 50cm3 of water into a 250cm3 beaker. Carefully transfer the bulk of the succinic acid from the weighing bottle into the beaker.
- Reweigh the bottle with any remaining succinic to the nearest 0.01g.
- Stir to dissolve the solid, adding more water if necessary.
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Transfer the solution to the volumetric flask (250cm3) through the filter funnel. Rinse the beaker well, making sure all the liquid goes into the volumetric flask.
- Add distilled water until the level is within about 1cm of the mark on the neck of the flask. Insert the stopper and shake to mix the contents.
- Then using a dropping pipette, add enough water to bring the bottom of the meniscus to the mark. Insert the stopper again but this time shake thoroughly ten times to ensure complete mixing.
- Clearly label the flask with contents, your name and the date.
The Titration Method Used:
Now a standard solution of succinic acid is available the titration can be carried out as follows:
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Using the funnel, rinse the burette with the succinic acid solution and fill it accurately with the same solution until the 50cm3 mark. Do not forget to rinse and fill the tip first though.
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Using the pipette filler, rinse the pipette with some of the sodium hydroxide solution and carefully transfer 25.0cm3 of the solution to a clean 250cm3 conical flask (Remembering to touch the bottom of the flask with the tip of the pipette for ten seconds to fully empty the pipette).
- Add 2-3 drops of the phenolphthalein indicator solution.
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Run the succinic acid solution from the burette into the flask, with continuous swirling, until the solution just loses the pink colour and goes clear. This first result may be used as a trial run because you will probably overshoot the endpoint on the first attempt. Record the final burette reading knowing that 50cm3 was the initial reading.
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Refill the burette with the succinic acid solution, again to 50cm3.
- Using the pipette, transfer 25.0cm3 of the sodium hydroxide solution to another clean conical flask. Add 2-3 drops of the phenolphthalein indicator solution.
- Carefully titrate the solution to a value near the end point of the trial run, slowing down and adding the acid drop by drop (with continuous swirling) when you think the colour is about to change.
- Repeat steps 5,6 and 7 at least until there are three concurrent results.
- Empty the burette and wash it carefully immediately after the titration, especially if it has a ground glass tap.
Use of Results:
If the 250cm3 of 0.1M sodium hydroxide solution contains 0.025 mols then by relating this value to ‘H2A + 2NaOH Na2A + 2H2O’ tells us that its going to take 0.0125 mols of succinic acid to completely neutralise the solution.
However we are only neutralising 25cm3 of sodium hydroxide so then whatever value is calculated for the number of moles via the formula ‘VC/1000’ must then be multiplied by 10.
e.g. If it takes 25cm3 of succinic acid to neutralise the solution then the molecular formula can be worked out as follows:
No. of moles = VC/1000 No. of moles = 25 × 0.05 ÷ 1000
= 0.00125 mols in 25cm3
= 0.0125 mols in 250cm3
Molecular mass = mass/no. of moles
Molecular mass = 1.65 ÷ 0.0125 132g
Therefore if the molecular mass of succinic acid is 132g then ‘n’ can be found by simply subtracting all but the (CH2)N from HOOC(CH2)NCOOH leaving:
(CH2)N = 42g
(14)N =42g
N = 3
Safety Precautions:
As with all work carried out in a laboratory certain safety precautions must be upheld. The first of these is safety glasses, which must be worn at all times during the experiment to protect the eyes from the toxic and corrosive chemicals involved. The second is when filling the burette and the pipette to do it properly with the right equipment (a filter funnel and a pipette filler respectively) to ensure there are no spillage’s and hence no skin contact with the corrosive chemicals being used. The last thing is a bit of common sense, making sure the burette is at a suitable height when filling it so that you aren’t stretching and therefore increasing the risk of spillage.
Accuracy of Equipment:
250cm3 flask: ±0.5cm3 (0.5÷250×100) = 0.2%
25cm3 pipette: ±0.05cm3 (0.05÷25×100) = 0.2%
50cm3 burette: ±0.15cm3 (0.15÷50×100) = 0.3%
Hence the total maximum percentage error for the experiment is 0.7%.
