In this experiment, trials 1, 2, and 4 will be used as they are considered accurate. Trial 3 will not be used and will be regarded as the inaccurate result. However, only the calculations of trial 1 will be shown in detail. The calculations for the other trials have been carried out in the same way and will be presented in tabular form.
Question: Calculate the concentration of the Sodium hydroxide solution.
Volume of Sodium hydroxide (NaOH)/cm3 (Vb) = 35.80 (±0.02 cm3)
– 9.25 (±0.02 cm3)
= 26.55(±0.04 cm3)
= 26.55±0.15%
Percentage error for Volume of NaOH = ±0.04 × 100%
26.55
= ±0.15%
Concentration of NaOH/mol dm-3 =? (Unknown)
Volume of Potassium hydrogen phthalate (Va) = 25.00 cm3 (±0.06 cm3)
= 25.00±0.24%
Percentage error for Volume of KHP = ±0.06 × 100
25.00
= ±0.24%
Concentration of KHP (Ma) = 0.10 mol dm-3
MaVa = MbVb
Vb = 26.55 dm3 (±0.15%)
1000
= 0.02655 dm3 (±0.15%)
Cb =?
Va = 25.00 dm3 (±0.24%)
1000
Ca =0.10 M
(N) = No. of moles
Na = Concentration ×Volume
= 0.10M × 0.02655 dm3 (±0.15%)
= 0.002655 moles (±0.15%)
Ratio of NaOH to KHP = 1: 1
Therefore, Nb = Na
So Nb = 0.002655 moles
Concentration = No. of moles
Volume
N = 0.002655 moles
V = 0.025 dm3 (±0.24%)
Therefore, Concentration of NaOH = 0.002655 moles (±0.15%)
0.025 dm3 (±0.24%)
= 0.1062 mol dm-3 (±0.39%)
Table showing the three accepted trials from the experiment, the volume of KHP from each trial, the concentration of KHP, Volume of NaOH used, the concentration of NaOH used, the average of the Volume of KHP solution and Concentration of NaOH solution.
Conclusion
The average concentration of NaOH derived from the calculations in this experiment is 0.1062 mol dm-3. The concentrations derived from each trial are very close to each other therefore the results could be valid. The major sort of error that affected my results was the calibrations on the burette – it was difficult to determine the volume of the NaOH. Since my results from the burette readings fall between 0.05 cm3, my results are therefore correct.
EvaluationIn this experiment, there were a number of errors that possibly occurred:
- Leaving the funnel on the burette while titrating.
Every drop in the experiment counts, by leaving the funnel on the burette, a drop or more of NaOH might get into the burette without one’s knowledge can increase the volume of the base.
In such a case as this, repeated trials while taking note of the position of the funnel will be the best way to reduce this error. This is a random error.
- Reading the wrong (upper) meniscus.
The upper meniscus does not give the exact reading on a burette instead it gives a reading higher than the actual level because it is not at eye level. Reading the upper meniscus could increase the chance of having a higher reading than the actual reading.
Always read the lower meniscus. It gives an exact eye level figure of the burette. Repeated experiments could also help in reducing this error. This is both a random and systematic error.
- Titrating without removing the air bubble from the burette tap.
Air bubbles increase the volume of liquid in the burette. Chances are that when there are air bubbles in the burette before titration, the results will have more inaccuracies.
Always check for air bubbles before starting the titration. If an air bubble is found close to the tap of the burette, it is advisable to open the tap to enable the air bubble escape. This is a systematic error so the burette should always be checked before use.
- Using the burette and pipette without first rinsing the burette with distilled water and then NaOH and the pipette with KHP.
The burette and pipette will contain some alkaline from the soap it is washed with. Using the burette without rinsing it could increase the alkalinity of the base (NaOH) and alter the results and add impurities to the solution. Using the pipette to transfer KHP without rinsing first could also change the acidity of KHP or alter its Ph.
Rinse the burette with distilled water first to remove some of the soap left from previous washing and then rinse it with NaOH to ensure cleanliness. Rinse the pipette with KHP before using it to transfer KHP. This is a systematic error. The apparatus should always be rinsed properly.
- Starting titration when the burette is less than half full.
If the burette is not filled enough, the NaOH might run out before the end – point is reached. In this case, time is wasted and the trial will have to be done again after refilling the burette.
Always make sure that the burette is full to at least 20.00cm3 before beginning titration.
- Working at different areas of a room with slightly different temperatures.
Difference in a room temperature could also cause results to be different. The higher the temperature difference in a room, the more likely it will be for results to be very different at different areas of the room.
Check the temperature in the room before and during the experiment to ensure that there is little or no difference in the temperature around the room.
- Titrating without any indicator.
Without the indicator, there will be no color change in the conical flask containing the acid and the base.
Ensure to add 2 or 4 drops of the indicator and swirl before beginning to titrate.
Limitations and weaknesses
- This experiment is restricted to an acid base neutralization. A precipitate cannot be used in this experiment.
- The experiment cannot be stopped at the equivalence point as there are no materials to do that. It can only be stopped at the end point.
- Leaving the funnel on the tip of the burette while titrating. This could increase the initial volume of the base making the result inaccurate.
- Forgetting to refill the burette after each titration to avoid running out of the base in the middle of the titration. In this case, that particular trial will have to be done again.
