(iv) Titration
The technique of slowly adding an acid to a base or vice versa-until the reaction has reached
the equivalence point is known as titration. The key to observing the equivalence point of a titration is the use of an indicator to indicate when the equivalence point of the reaction has been reached. Indicators take many forms. The most familiar indicators are litmus and phenolphthalein. Litmus is a vegetable dye that is red in acid and blue in base. Phenolphthalein is colourless in acid and pink in base. In practice, each indicator has an endpoint-at which it turns colour-that might be slightly different from the equivalence point of the reaction. Titrations that use phenolphthalein as the indicator, for example, should be stopped just before the solution turns a permanent pink colour. (1) Lewis, R., Evans, W., 2011
Materials:
The chemicals that were used in this experiment were phenolphthalein indicator, water, hydrochloric acid, sodium hydroxide and potassium acid phthalate.
The apparatuses that were used in this experiment were burette, beaker, pipette, analytical balance, dropper, conical flask, glass rod, and volumetric flask.
Methods
1. Weigh accurately mass approx. 5 g of potassium hydrogen phthalate (KHP) into a weighting boat record the mass to the three or four decimal place.
2. Transfer to a beaker and add distilled water. Swirl the baker or with glass rod until the solid is completely dissolved.
3. Carefully place a stirring magnet into 250cm3 volumetric flask and make up to mark with distilled water and shake well.
4. Pipette a 25cm3 amount of this into a conical flask. Add two or three drops of phenolphthalein.
5. Now fill the burette with the sodium hydroxide solution and remove any air bubbles from the tip of the burette.
6. Take the initial burette reading.
7. While stirring the conical flask slowly add the sodium hydroxide from the burette until the solution in the conical flask turn pink. Try to stop it at the first hint of faint pink. Record the final volume of the burette.
8. Repeat this procedure until consistent result
Results
Table 1 - shows the titration of sodium hydroxide into 250cm 3 of potassium hydrogen phthalate solution.
Experiment
Initial volume of burette (cm3)
Final volume of burette (cm3)
Final titration NaOH (cm3)
rough
0.20
26.20
26.00
1
8.20
34.10
26.00
2
1.30
27.60
26.30
3
2.90
28.90
26.0
Calculation
(a) Your weighing (to four decimal places), and all titration results in a table. Remember to number and title the table.
Mass of potassium hydrogen phthalate used = 5.0008 g
5.0008 g of potassium hydrogen phthalate is diluted into distilled water and solution is made up to 250cm 3 this potassium hydrogen phthalate is used titration.
(b) The calculation of the relative molecular (Mr) mass of potassium hydrogen phthalate (HKC8O4H4).
Mr of potassium hydrogen phthalate (HKC8O4H4) is 204.1
(c) The calculation of the molarity of the solution of potassium hydrogen phthalate.
Moles = mass / relative molecular mass
Moles of HKC8O4H4= 5.0008g / 204.1 = 0.0245mol
Molality = moles / volume
Molarity of HKC8O4H4 = 0.0245mole / ( 250cm3 / 1000) = 1.568 moldm -6
d) Molarity calculation of sodium hydroxide solution
Molar ration of HKC8O4H4 to NaOH is 1.1. Therefore the mole of HKC8O4H4 are the same as NaOH which is 0.0245mol.
Molartiy of NaOH = 0.0245mol/ (26.00cm3 / 1000) = = 0.95moldm3
Discussion
Conclusions
By carrying out this experiment a key finding was established of example the amount of so hydroxide solution required to provide an experiment neutralise a 25.00cm3 of potassium hydrogen phthalate solution. The experiment results had been repeated over three times to allow the results to be as accurate as possible, factors which could limit the experiment; such as the meniscus line on the burette could have been misread which led to obtain inaccurate volume of NaOH used in the experiment. Also the color changes within an indicator could affect the outcome of the end reaction lead to a false reading with the burette. The results were used to calculate the molarity with both solutions. During any experiment it’s important to reduce and minimise errors one way would be to repeat experiment over many times as possible. Also this will provide accurate results as there will be more comparative within the experiment, by carrying repeats this will allow the experiment to become reliable and valid. The experiment overall was successful as the aim of this experiment was achieved within a safety environment.
1. Lewis, R., Evans, W., 2011. Chemistry Palgrave foundations.4th ed. Hampshire: Palgrave Macmillan.
