Key variables:
mx – weighted amount of KHC8H4O4 in each trial
Vx – volume of NaOH solution in each trial
CNaOH – exact concentration of NaOH solution in each trial
Planning (b)
Requirements:
- 1 burette (25 ml)
- 2 beakers
- 3 calibrated flasks (500 ml)
- 1 plastic bottle (1500 ml)
- 0.5% solution of phenolphthalein
-
0.1 mol dm-3 NaOH (1 dm3)
- balance
- lime water
Method:
Titrating the solution of NaOH over potassium hydrophtalane to calculate the exact concentration of the solution of NaOH.
Procedure:
-
I measured accurately about 5 g of sodium hydroxide and dissolved it in 1 dm3 of lime water.
-
On the analytical balance I measured 0.5-0.9 g of potassium hydrophtalane and dissolved it in 70 cm3 of water.
- Subsequently I added 4 drops of 0.5% solution of phenolphthalein and titrated it using water solution of NaOH until pink colour appeared.
- The above procedure was repeated 3 times to obtain 3 results.
Data Collection
HCl + NaOH → NaCl + H20
mNaOH = 5 g
Vsolution = 1 dm3
CNaOH = 0.1 mol dm-3
Mpotassium hydrophtalane = 204.22 g mol-1
CNaOH = mx / Vx Mpotassium hydrophtalane
mx – weighted amount of KHC8H4O4 in each trial
Vx – volume of NaOH solution in each trial
Mpotassium hydrophtalane – molar mass = 204.22 g mol-1
In the equations below molar mass of potassium hydrophtalane is shortened to Mph
Data Processing & Presentation
C1 = m1 / V1 Mph = 0.791g / 32.2 cm3 * 204.22 mol = 0.12028 g mol dm-3
C2 = m2 / V2 Mph = 0.903g / 36.8 cm3 * 204.22 mol = 0.12015 g mol dm-3
C3 = m3 / V3 Mph = 0.528g / 21.5 cm3 * 204.22 mol = 0.12025 g mol dm-3
Errors and uncertainity
- Balance: 0.14 %
0.001g / ((0.791g + 0.903g + 0.528g) / 3) * 100% = 0.14%
- Pipette: 0.16 %
V = 25 cm3 → 0.04 cm3
0.04 cm3 / 25 cm3 * 100% = 0.16%
- burette: 0.5%
- calibrated flask: 0.2%
Total: 1%
Conclusion & Evaluation
The advantage of using potassium hydrophtalane as an alcalimetric primary substance is its relatively high molar mass.
The hypothesis of this experiment (If the mass of potassium hydrophtalane and the volume of NaOH used in each trial are known, we may find out an exact concentration of NaOH solution) worked well. The experiment was rather a simple one. The purpose of it was to make us understand and apply titration in practice and not only theory.
Although observing the exact moment of changing the colour of the solution from orange to yellow may be difficult and misleading, the results confirm that the substances were measured accurately.
Errors made in this experiment should not have exceeded 1% of total mass of the substances. As far as classroom conditions are concerned it is a good result. This lab was not very complicated and there were not any possibilities to have the results upset i.e. by emitting gas or heat. It would be better, however, if the burettes worked from the very beginning and were airtight.
Sources:
- Green J, Damji S. 2001. Chemistry. Second edition. IBID Press, Victioria, Australia.
- http://www.answers.com/topic/solution
- http://www.dartmouth.edu/~chemlab/techniques/titration.html
