To prepare my acid solution I must dissolve an accurately weighed amount of anhydrous succinic acid to 250cm³/0.25dm³ of water to achieve a concentration of 0.1mols/dm³. What mass of acid is required to achieve this concentration?
n = c × v which is 0.1 × 0.25 = 0.025 moles
The mass of 0.025 moles = n × Mr which is 0.025 × 132 = 3.3g
This is obviously not an accurate mass as I do not know the real Mr but this is an estimate so that the reaction occurs at a suitable rate.
I can now continue with the preparation-
Equipment requirements include:
- Distilled water in wash bottle
- Small funnel
- Weighing boat
- Spatula
- Standard (250cm³) graduated flask
- Beaker
Firstly the graduated flask and beaker must be rinsed with distilled water to prevent impurities in the acid solution which would affect the results.
Accurately weigh out 3.3g succinic acid.
Initially dissolve the acid in a minimum amount of distilled water in a beaker, stirring with a glass rod.
Transfer all the acid into the graduated flask. The best way to ensure all is transferred is to place the funnel on top of the flask and wash the solution in with a jet of distilled water from the wash bottle.
Continue to add distilled water up to the 250cm³ mark. The base of the meniscus should be level with the mark to achieve the maximum accuracy of results.
Once this has been completed I can proceed with the titration. In this experiment I will use a solution of Sodium Hydroxide to standardise my solution of Succinic acid.
Equipment requirements include:
- Distilled water wash bottle
- Sodium Hydroxide (0.1mols/dm³)
- Succinic acid (0.1mols/dm³ approx.)
- Phenolphalein indicator
- Burette (50cm³) and stand
- Conical flask (250cm³)
- Pipette (25cm³) and filler
- Small beaker and funnel
It is a good idea to perform a rough titration to get an idea of where the end point will be.
Firstly all the equipment must be thoroughly rinsed with distilled water to minimise the risk of impurities affecting the reaction.
Then fill the burette with acid up to the 0 mark. This should be done below your head for example on a stool to prevent an accident.
Pipette 25cm³ of the sodium hydroxide solution into the conical flask and add two drops of indicator.
Begin to slowly add the acid, continually swirling the mixture. As you near the end point add the acid drop wise so you know exactly when the reaction is complete and consequently gain more accurate results.
When the whole solution has just turned completely pink then the reaction is complete and no more acid should be added.
Record the volume of acid that has been added clearly in a table.
Repeat the experiment until you obtain at least two results that are within 0.1cm³ of each other.
In 25cm³/0.025dm³ of Sodium Hydroxide solution of concentration 0.1M the number of moles is:
n = c × v = 0.1 × 0.025 = 0.0025 moles
The equation
H A + 2NaOH Na A + 2H O
Tells us that the acid and alkali react in a 1:2 ratio
So if the NaOH solution containing 0.0025 moles was neutralised by 0.0236dm³ of the acid then this volume of acid must have contained 0.0025 = 0.00125 moles
2
So the real concentration of the acid solution = n = 0.00125 = 0.053mols/dm³
v 0.0236
If 0.25 dm³ succinic acid solution contains 3.3g succinic acid then 3.3g contains
n = c × v = 0.053 × 0.25 = 0.0133 moles.
Therefore Mr = m = 3.3 = 248
n 0.0133
Molecular formula – what is n ?
1 + 16 + 16 + 12 +12 + 16 + 16 +1 = 90
All the molecules of (CH )n = 248 – 90 = 158
One molecule of (CH )n = 12 + 2 = 14
No. of molecules of (CH )n = 158 ÷ 14 = 11.29
So the formula = HOOC(CH ) COOH
Accuracy and limitations of equipment
There are limitations to the apparatus and methods I have used. Liquids may remain ‘stuck’ to the sides of tubes, beakers and other glass apparatus. It is also impossible to remove all the solid from one container when transferring it to another. Completion of the neutralisation reaction is detected by a colour change seen by the human eye. The reaction is complete when the whole solution JUST turns pink. It is very difficult to identify this exact point by this method so results can not be recorded to a very accurate degree – it would be pointless measuring to 5 d.p. if your method of recording is not that reliable. Errors can occur with measurements using certain apparatus, the following are the maximum errors you may incur:
1dm³ standard flask ± 0.80 cm³
250cm³ standard flask ± 0.30cm³
25cm³ pipette ± 0.06 cm³
50cm³ burette ± 0.10cm³
The value is the difference between two points for example using a 250cm³ flask you might measure to 250.1cm³ or 249.8cm³, a difference of 0.30cm³.
Potential hazards and safety precautions
Experiments involving acids and alkalis are always dangerous as both are corrosive but we can minimize the risks by following safety precautions.
- Always wear safety goggles when near chemicals
- Wear gloves when handling vessels containing acids or alkalis as both are corrosive, the severity of which depends on the concentration of the acid/alkali
- When filling the burette bring it to below eye level to eliminate the risk of tipping acid on your head
- Never ‘suck up’ a solution when filling a pipette
- Don’t low the last drop out of the pipette, the alkali it contains in this experiment is very harmful
- Don’t eat or drink around areas where acids or alkalis have been used. Don’t eat after handling a vessel containing an acid or alkali without first washing your hands.
