[earthnation112]

Just wanted some input and guidance on my answers if they were correct or not. The questions are as follows:

(b)Two 0.0400 M acid solutions were prepared (labelled A1 and A2). One of them was a weak acid and the other was a strong acid. The pH of A1 was 4.52 and of A2 was 1.40. Assume that all activity coefficients are equal to 1.

(c)   25.00 ml of each of the acids in part (b) was titrated with 0.1000 M KOH.

(i)    Sketch the pH titration curve expected for A1.
(ii)   Sketch the graph expected for a conductimetric titration of A1.
(iii)   On all sketches carefully label the axes and mark the end points.)

So I know that A1 is a weak acid and KOH is a strong base, my attempt for the pH titration curve and the conductimetric titration for A1 have been attached below.

Are my attempts correct? Have I done the following correct:

1.   Have I used the correct titles for the axis for both graphs?
2.   The ph of A1 is 4.52, I placed this on the starting point of the line for the graph of pH titration, is that correct? 
3.   25 ml of KOH was used, is it correct then to end the x axis with 25 at the end like I have done?
4.   Are the end points indicated for both graphs done correctly?
5.   Is there anything I can add to my graphs to make them clearer?

The reason I am asking all these questions is because I am going off the notes provided by my teacher which don’t appear clear to me as I have never done the topic before so I just used the sample graphs presented without no titles or the like and had an attempt. Hence the reason for asking about some questions which seem very obvious like the one's referring to the title of the axis.

Any input would be greatly appreciated, thank you.

[Burner]

I am not really familiar with titrations curves and also haven't learnt the term 'activity coefficient', but according to my basic knowledge in titrations:

1.   Have I used the correct titles for the axis for both graphs?
2.   The ph of A1 is 4.52, I placed this on the starting point of the line for the graph of pH titration, is that correct? 
3.   25 ml of KOH was used, is it correct then to end the x axis with 25 at the end like I have done?
4.   Are the end points indicated for both graphs done correctly?
5.   Is there anything I can add to my graphs to make them clearer?

1. Yes
2. Yes (From the statement each of the acids in part (b) was titrated with 0.1000 M KOH, the acids are placed in the conical flask and KOH is in the burette)
3. Yes
4. Not sure (Shouldn't the end point of the pH-volume graph be located at pH=7?)
5. Not sure

Please correct me if I had made any mistakes.

[Borek]

3.   25 ml of KOH was used, is it correct then to end the x axis with 25 at the end like I have done?

Why do you think 25 mL of KOH was used? What was the initial acid concentration?

The weak acid titration curve looks incorrect to me. In general shape of a weak acid titration look different (it initially goes a bit up), then, initial pH that you have allows you to estimate acid Ka, and we know pH=pKa at the titration midpoint, your curve doesn't use this information. Your curve is way too low and has wrong initial shape, at the same time it starts at a correct pH and you are right about the endpoint being above pH 7. Hard to say whether it will be accepted or not.

Attached titration curve is that of the acetic acid - it shows you the correct shape.

[earthnation112]

3.   25 ml of KOH was used, is it correct then to end the x axis with 25 at the end like I have done?

Why do you think 25 mL of KOH was used? What was the initial acid concentration?

The weak acid titration curve looks incorrect to me. In general shape of a weak acid titration look different (it initially goes a bit up), then, initial pH that you have allows you to estimate acid Ka, and we know pH=pKa at the titration midpoint, your curve doesn't use this information. Your curve is way too low and has wrong initial shape, at the same time it starts at a correct pH and you are right about the endpoint being above pH 7. Hard to say whether it will be accepted or not.

Attached titration curve is that of the acetic acid - it shows you the correct shape.

The reason I think 25 ml of KOH was used Is because the question said "25.00 ml of each of the acids in part (b) was titrated with 0.1000 M KOH. " I made a mistake, since 25ml was referring to the volume of weak acid used and not the volume of KOH, so should the x axis just have the title "0.1000 M KOH" since only the molarity of KOH is given and not the actual volume.

My teacher gave an example of four graphs, one for strong acid/strong base where the ph at the end point is 7. For weak acid/strong base the ph end point is greater than 7. For strong acid/weak base pH end point is less than 7 and finally for weak acid/weak base the pH end point is close to 7.

I literally used the sample graph that was given for weak acid/strong base and copied it and inputted my numbers and added some extra detail. I have attached all the four graphs below to show the examples that were provided to me regarding this topic.

[Borek]

25ml was referring to the volume of weak acid used and not the volume of KOH, so should the x axis just have the title "0.1000 M KOH" since only the molarity of KOH is given and not the actual volume.

You can easily calculate volume of KOH needed for titration.

My teacher gave an example of four graphs, one for strong acid/strong base where the ph at the end point is 7.

OK. These are rather sloppy plots, but the only person that can be blamed for their sloppines is your teacher

Still, I would shift the left part of the plot up. Try to evaluate the pH at midpoint, you will see that the plot is wrong by several units.

[earthnation112]

To calculate the volume of KOH used I used the following formula:

MaVa = MbVb

Ma = 0.0400M
Mb = 0.1000M
Va = 25m
Vb = ?

Rearranging the formula to get Vb was:

Vb = MaVa/Mb
So Vb equals 10 ml

From what I understand, I end the x axis with 10ml instead of marking 10ml on the x axis at the pH end point? Is that accurate? Thank you for the input and help.

       

[Borek]

To calculate the volume of KOH used I used the following formula:

Why don't you just use simple stoichiometry, instead of "formula"?

So Vb equals 10 ml

From what I understand, I end the x axis with 10ml instead of marking 10ml on the x axis at the pH end point? Is that accurate? Thank you for the input and help.

If you end the x axis at 10 mL, you will see the curve just to equivalence point. Go to twice that.

[earthnation112]

So as a general rule, should I always end the x axis at double the volume of the base used? In this example the volume of base calculated was 10ml if it was 20ml would I end the x axis at 40 ml? Thanks

[Borek]

This is a bit more tricky. In the case of diprotic acid, if the equivalence point is at 10 mL, it is enough to draw curve up to 15 mL - everything interesting will be already covered.

[earthnation112]

What's a good general rule to apply in my case? Because the four graphs I have attached are the sample graphs given to me, I would need to copy these graphs exactly and then just insert the information provided by the question given.

So would it make sense in that case to simply double the volume of base used?

[Borek]

So would it make sense in that case to simply double the volume of base used?

As you are asked about monoprotic acid - yes.

[earthnation112]

Thanks a lot, the input really helped.