Hey,
I have a titration practical coming up to determine the ethanoic acid content in a vinegar sample.
In the marking criteria it asks for "specific equations for the reactions that occur during the lab," but i only know of the neutralisation reaction. What could the other possible equations be?
Also for the conclusion, it asks to "identify area(s) of the lab most likely to cause experimental error and propose solutions to problems encountered in the investigation." I have asked my teacher about errors such as Naoh being hygroscopic and altering the results, the end point not being the same as the equivalence point and how colour changes of the indicator are not instant, I have also asked about the indicator changing the pH of the solution and also how each individual can have a unique colour perception when determining the end point. But, my teacher has rejected all of those suggestions and also said no human errors and equipment errors can be written. What other possible errors and improvements are there? Am i misreading something from the criteria?
Also, what should i write about the validity and accuracy of the experiment?
-Thanks
Hey Loki!
Reading your description, I'm assuming that you are conducting a titration involving acetic acid and NaOH? Ok, then let's get started!
When the teacher asks you for equations, you need to know 3 equations: (for convenience sake lm not gonna write in states, but you should write them in an exam)
1.
The Balanced chemical equation (CH
3COOH + NaOH ----> NaCH
3COO + H
2O)
2.
The net ionic equation (CH
3COO
- + Na
+ ----> NaCH
3COO)
3.
Hydrolysis equations of salt (CH
3COO
- + H
2O ---> CH
3COOH + OH
-)
Equation 1 and 2 are pretty straight forwards. Equation 3 is also significant because it shows why the endpoint would be slightly basic. Common exam questions would ask you for whether the endpoint is basic, acid or neutral and would ask you to identify a suitable indicator. If your teacher wants to differentiate between band 5 students from band 6 students, they would ask you why the endpoint would be basic. In this case, referring to equation 3 from above, since hydroxide ion is given off, the endpoint would hence be basic.
With your second question identifying the possible errors in the experiment, I would personally have written a human error or equipment error (it's kinda weird how your teacher is completely rejecting these errors). But look if your teacher says none of these can be included, then we have to find some other flaws in the experiment. One thing you can perhaps talk about is impurities from the atmosphere entering the conical flask or the burette and this can cause slight differences. Another thing maybe carrying out titration at the wrong temperature, which would affect experimental outcome because some indicators are quote sensitive to surrounding temperature. I'm a little confused by the meaning of "area(s) of the lab", because Im not too sure whether your teacher is talking about the experiment or the physical places in the lab that would cause error. Can you please enquire the teacher and come back to us on that? That would be helpful.
With the validity and accuracy part its quite easy, I will list some things for you to consider and you can write it out in a formal manner:
Validity:
- the extent to which the experiment tests the hypothesis
- Is there only one independent variable involved? If yes, then the experiment is valid
- Does your methods answer the aim? If yes, then the experiment is valid
- Is there a very significant difference between your result and your hypothesis? If yes, then the experiment is invalid. (This may sound like a point for accuracy, but accuracy is only concerned with relatively small deviations from your hypothesis. A significant deviation would mean that you are doing something wrong)
Accuracy:- Did you thoroughly clean your equipments? Did you do it for multiple times? If yes, accurate
- Are your instruments correctly calibrated? If yes, accurate
- Is your weighing instrument presenting a precise quantity? (i.e. the value it displays does not fluctuate) If yes, accurate
- Are you biased in your reading of the scale? If yes, inaccurate
- Is the scale on your pipette and burette correct? If yes, accurate
- Are you reading from the bottom of meniscus (i.e. parallax error)? If yes, accurate
Then talk about how to minimise the impact of these inaccuracies if they exist.
Hope my answer helped! Remember to come back to us after you have enquired the teacher about that second point!
Best Regards
Happy Physics Land
- Are all other variables controlled? If yes, then the experiment