Preliminary results
We took some preliminary results to check to see if our method works and to make any changes before doing the proper results.
Apparatus - Preliminary
- Vinegar
- Burette
- Clamp stand
- Standard flask (100 cm3)
- Standard sodium hydroxide solution ( 0.4M )
- Phenolphthalein indicator
- Pipette
- Conical flask (100 cm3)
- Wash bottle/distilled water
- Flask for waste NaOH
We washed the burette out with NaOH so that there would be no impurities inside the burette that could cause inaccurate results. If the was water inside the burette this would have weakened the NaOH.
We measured out 10cm3 of the vinegar we were going to experiment with using a pipette. and dropped this into a conical flask.
We then set up the apparatus as shown above with the conical flask containing the vinegar and phenolphthalein indicator underneath the burette containing the NaOH. We then added the NaOH to the conical flask until the indicator changed from clear to pink. We then knew that we had reached the stoichiometric point. We took the result and then refilled the burette and repeated the process. For the preliminary results we only used to vinegars. These were dilute distilled malt vinegar and dilute malt. All the vinegars we used had been diluted 10 times so we would have to multiply our results by 10.
Results
We saw that both of the vinegars were very close together. This suggested that we needed to change our experiment in some way. We then realised that by using a weaker concentration of NaOH (0.1 M) that it would take longer to change so the results would be more accurate. We also realised that we could add a certain amount of NaOH to the solution safe in the knowledge that it wouldn’t meet the stoichiometric point. This speeded the whole experiment up. We also realised that the distilled malt vinegar had more Ethanoic acid. We believe that because it has been distilled, more sugar has turned to alcohol and therefore more alcohol has turned into Ethanoic acid. We know that NaOH is in a one to one reaction with the Ethanoic acid:
NaOH + CH3COOH ⇒ CH3COONa + H2O
This means the amount of NaOH is the same as the amount of acid.
MaVa / MbVb = 1
This means we can write this as MaVa = MbVb
Ma = 0.4
Va = 2.0
Mb = x
Vb = 10
0.4 X 2.0 = 10x
0.8 = 10x
0.08 = x
We then multiply this answer by 10 as the vinegars were diluted 10 times. This means that the amount of Ethanoic acid in the Dilute malt, according to our first test.
10(0.4 X 2.1) = 10x
10(0.84) = 10x
0.84 = x
Practical
We decided to use the same set-up for the practical as we did for the preliminary. The only thing we changed was the strength of the NaOH. We changed this to 0.1M as this would give us a more accurate reading.
Results
Calculations
From this we can work out the percentage of acid in the vinegar. We have to first work out the relative formula mass of Ethanoic acid. To do this we just add up all the atomic weights in the formula CH3COOH.
C = 12. 01
H = 1.01
0 = 16
2 X C
+
4 X H
+
2 X O
= (24.02) + (4.04) + (32) = 60.06
The calculation for the percentage of acid in the vinegar is as follows:
Ethanoic acid content multiplied by 60.06 (relative formula mass of Ethanoic acid). We then divide this by 1000 as it was in 10 ml. Then to get a percentage we need to multiply by 100.
Example:
0.86 * 60.06 = 51.6516
51.6516 / 1000 = 0.0516516
0.0516516 * 100 = 5.17 % (3 S.F)
Results table
Evaluation
To help us with the evaluation we can construct some graphs.
Graph to show Ethanoic acid content in percent
This shows us that Red wine vinegar and distilled vinegar are higher in Ethanoic acid than the rest. To find out why we have variations in the amount of acid in vinegars we need to look closely at how it is made.
Cider, as an alcoholic drink contains roughly 3 % - 4 % alcohol. This is fairly low which explains why cider is at the bottom of the table. Red wine contains about 9% alcohol but can contain up to around 15%. This will explain why red wine is at the top of the table. White wine is normally slightly lower in alcoholic content than red wine. This is also reflected in the table.
Experimental error
In all experiments there are a number of experimental errors which could cause inaccurate results. One of the biggest mistakes could have been the reading of the amount of NaOH used.
In the case above we can’t tell where to take the reading from. The top of the reading is clearly at 1.05 but the bottom of the reading is nearer to 1.1. As a standard we always took the reading from the top of the curve. This could have happened at many points during the experiment. These points include
-
measuring out the 10 cm3 of vinegar.
- Taking the reading form the burette.
The NaOH was added in drops until the indicator changed colour. When we added the NaOH at the start we added it as a constant stream. When we were reaching the end of the titration we added it by drops. If the drops were added to quickly and the acid and the base hadn’t reacted then it was possible that we added too much of the base.
