CH3COOH (aq) + NaOH (aq) → CH3COONa (aq) + H2O (l)
Acid and base are electrolytes, so when they dissolve in water they dissociate and produce ions. The acid produces H+ ion and the alkali produces OH- ion.
Equation:
H+ (aq) + OH- (aq) → H2O (l)
Ethanoic acid is a weak acid so it has less H+ ions as less of the acid molecules disassociate to form ions. Which means more of it is needed to neutralized Sodium hydroxide which is a strong alkali. Because NaOH has lots of OH- ions and need as many H+ ions to form water. The less volume of acid need to neutralise the 25cm3 of sodium hydroxide, the more concentrated it is, because the same amount of H+ ions will still be needed.
No. of moles = concentration × volume
The volume of NaOH is 0.025dm3
The concentration of NaOH is 0.1moles/dm3
0.025 × 0.1 = 0.0025 moles
We know that the molar ratio of the acid to the alkaline from the equation is 1:1, so the no. of moles of the acid also is 0.0025. We know that the maximum volume of solution the burette can held is 50 cm3 or 0.05 dm3,
C = N / V
C = 0.0025 / 0.05
C = 0.05 moles/dm3
If the burette isn’t refilled, the concentration of the vinegars need to be 0.05 moles/dm3 or more.
Plan:
- Set up the equipments.
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Use the pipette filler to fill sodium hydroxide into a pipette up to the labelled line which is 25cm3. This is done by putting the pipette into its filler and by rolling the scroll on it down to draw in solution, roll the scroll around to adjust the level and get the exact amount on the line.
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Once the pipette is filled, place it into a conical flask; remove the pipette filler from the pipette to let the sodium hydroxide solution run down into the conical flask.
- Add just a few drops of Phenolphthalein which will turn the solution pink, because too much will make the test inaccurate.
- Make sure the burette is held firmly by the burette stand and its tap closed tight, pour the acid to fill the burette up to the point 0, and make sure there are no air bubbles. Read off the meniscus at eye level by the bottom of the meniscus.
- Add the acid into the conical flask by opening the tap on the burette a little each time while swirling the flask at the same time to mix them evenly, when the solution gets nearly colourless, just let one drop fall each time until the solution is just neutral, when it turns colourless.
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By reading off the scale to the accuracy of 0.05cm3 on the burette, find out the exact amount of the different acids need to neutralise 25cm3 of sodium hydroxide.
- Repeat the steps 3 times each for different vinegar, until you get 3 results within 0.1 concordant of each other for each vinegar, to get more accurate and reliable results.
Safety:
Fair test
To ensure that it’s a fair test by controlling the variables, keep the apparatus used unchanged, use the same amount and concentration of sodium hydroxide, and do the experiment under the same condition and temperature.
Accuracy
To ensure all the measurements are as accurate as possible, read off the burette and pipette at eye level to the bottom of the meniscus.
Results:
- White wine vinegar
- French cider vinegar
- French white wine vinegar
- Rice vinegar
- Malt vinegar
Evaluation
My results were fairly reliable as I repeated each test many times to get 3 results within 0.1 concordant. But I carried my experiment over different days. I was able to read off the burette at an accuracy of 0.05 cm3 to the bottom of the meniscus, but it was quiet difficult to read the burette at eye level as I had to stand on tip toes.
The concentration of the acid was diluted to 10% of its original strength to ensure the accuracy of the results. This is because the more diluted it is, the bigger volume it would to neutralize the alkali, therefore the ratio of the inaccuracy compare to the total volume needed be smaller, which means the accuracy increases by 10 times.
I had a few anomalies and the were all a bit too high than the rest of my results, this is probably due to the fact that I didn’t close the tap of the burette in time as it was quiet hard to control, so extra amount of acid were dropped in, or I might not have read off the burette at eye level. Also I might not have got the right amount of the alkali, because it was quiet hard to use the pipette filler and get it to the exact volume, and some of the alkali might have spilled out of the pipette in the process to place it in the conical flask. There is also a possibility that the vinegars which we replaced after the original ones ran out had different concentrations as they were of different batches, so this also made a difference the my results.
To improve my procedure, I could increase the volume of the alkali used, and dilute the acids more to 5%of its original strength for even more accurate results.
For further investigation, to improve the reliability of my results and the validity of my conclusion, I could extent this experiment by using a wider range of acids, rather than just using vinegars. As fruits are also acids, I could use orange juice, apple juice, lemon juice etc. and I could even use soft drinks and tea for the acids.
