An investigation to compare and determine the concentrations of some alkalis in saturated solution.

Apparatus

The following apparatus must used in order to complete this investigation:

• Four 100cm3  beakers
• 250cm3  conical flask
• 250cm3  volumetric flask
• 50 cm3 burette
• 25cm3 glass pipette
• Pipette filler
• Plastic pipette
• Weighing balance
• Spatula
• Glass rod
• White tile
• Funnel
• 1 bottle of distilled water
• Lab coat
• Safety goggles
• Gloves
• Burette stand

Method

• Construction of the primary standard solution

1. Place the bottle containing the base (solid NaOH) on the weighing balance and record the weight.
2. Dissolve the solid into 100cm3 distilled water and mix with the stirring rod until all of the solid has completely dissolved.
3. Now re-weigh the empty bottle and record the weight
4. Work out the weight of the actual solid transferred. This can be done by subtracting the weight of the empty bottle from the total weight (bottle and contents).

The readings taken via the above steps should be recorded in this table of raw data:

The data from above can be used to work out the number of moles of Ethanedioic acid.

• Titration procedure

In order to help give us an idea of what colour the end-point actually is, the following small experiment could be followed so that we can recognise the end-point more quickly.

• In a small beaker, mix 3cm3 of Ethanedioic acid with 3cm3 Sodium hydroxide solution.
• Add 4 drops of phenolphthalein indicator and mix together.
• A faint pink colour should be achieved.
• This is the actual colour that should be present at the end-point of the titration

It is recommended that a trial titre should be carried out before hand in order to determine how much acid is needed to give an equivalence point of the solution. This is the point where the base is neutralized.

Following this, a set of three or four accurate titres should be carried out. To ensure accuracy of the end-point and to make sure that it is not missed, it is important to add the acid from the burette in drops.

1. Pour the primary standard solution prepared earlier into the volumetric flask using the funnel to avoid spillage.
2. Add more distilled water to the solution in the volumetric flask to make the total volume 250cm3
3. Put the stopper of the flask on it and shake vigorously a good few times to make sure that all the contents are dissolved thoroughly.
4. Label this solution
5. Using the pipette filler, fill the glass pipette with Sodium hydroxide solution from the volumetric flask
6. Release the contents of the pipette into a conical flask and add 4 drops of phenolphthalein indicator
7. Fill the burette fully with Ethanedioic acid, making sure that the tap is closed to prevent the leakage of the acid, and with the miniscus at eye level reading 0
8. Record this initial reading
9. Record all data to two decimal places.
10. Open the tap of the burette and titrate the acid with the solution. At the same time, swirl the flask
11. Continue to titrate but take extra care when approaching nearer to the end-point.
12. When the end-point is near, start to release the acid from the burette in drops. Allow 1 or 2 drops into the conical flask at a time remembering to swirl the flask after each addition
13. When you reach a permanent change in colour, this is the end-point which should be a faint pink colour
14. In order to obtain more accurate and reliable results, repeat steps 5 to 13.
15. Using the same method, carry out this experiment for the other metal hydroxides. (Calcium, potassium and magnesium hydroxides).

Results of the titration of Na(OH)2

               

                                       To calculate mean titre:

   

                titre 1  +   titre 2  +  titre 3

              3

• This mean titre represents the accurate volume of Sodium Hydroxide needed to neutralise 25cm3  Ethanedioic acid

1.                        Using balanced equation, deduce the number of moles of base from                        

       H2A  +  Ca(OH)2         2H20  +  Na2A

         Substitute the values of data into this formula:    n =  c  X  v

                                                                                           

            1000

This is so to make ‘c’ the subject of the expression in order to calculate the concentration of Sodium hydroxide.

        C = 1000     X   n                  n = number of moles of Sodium Hydroxide

          v        v = volume of Sodium Hydroxide

This will give us the concentration of the Sodium Hydroxide…mol.dm3 

• Repeat the above calculation steps for the other metal hydroxide titrations.

Risk Assessment

The following health and safety aspects need to taken into account when doing this experiment:

• Lab coats must be worn during the experiment as the chemical are corrosive and can permanently damage your clothes.
• The Ethanedioic acid is very dangerous and harmful. Therefore eye protection, such as safety goggles, must also be worn to protect the eyes from harmful chemicals.
• Long hair must be tied back
• Bottles must be kept away from the edge of the bench to avoid spillage
• Certain apparatus should be handled with extra care, such as glassware. This applies especially when they are filled with contents
• Make sure that the tap of the burette is properly closed when filling it, to avoid leakage of the acid.
• Fill the burette using a funnel to avoid spillage
• All spillages should be mopped and cleaned up immediately to prevent damage and injury to others

Reliability and Precision

• To ensure accurate quantities of acid and hydroxide solutions, a Gilson pipette could be used
• The inside of the burette must be thoroughly rinsed to avoid dilution or increased concentration of the acid.
• The pipette should also be rinsed
• Ensure that all the contents from the pipette is emptied into the conical flask. If not, this could lead to inaccurate volume being used
• When reading off the burette, make sure you are at eye level with the miniscus
• When recording the readings, record to two decimal places
• The end-point can also be determined using a colorimeter, just to ensure further accuracy
• It is recommended to repeat each experiment at least 3 to four times