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Unit-4 - Which (Acid-Base) indicator should I use?

Titrating ethanoic acid with 0.1mol dm-3 sodium hydroxide

Can someone tell me step-by-step on how to solve this question?

NOTE: I have the data booklet (A-level U4)

Reply 1

Zahid~

Is there no other information in the question?

Reply 2

ihaspotato

Original post by Zahid~

Is there no other information in the question?

No, it's a MCQ question.

A) Thymol blue (acid)
B) Bromothymol blue
C) Thymol blue (base)
D) Alizarin yellow R

Reply 3

AvWOW

For strong acids it's commonly Phenolpthalein. Mainly cause estimated pH range for strong acid weak base graph a vertical region between around 7-10.5 is given, and the pH range of the indicator should correspond to the vertical region.
But if further information is given, we can give a more accurate indicator.

Reply 4

AvWOW

Original post by ihaspotato

No, it's a MCQ question.

A) Thymol blue (acid)
B) Bromothymol blue
C) Thymol blue (base)
D) Alizarin yellow R

Is Thymol blue (base) the answer?

Reply 5

ihaspotato

Original post by AvWOW

Is Thymol blue (base) the answer?

CORRECT! can you please tell the steps you did to get to that conclusion?

(edited 7 years ago)

Reply 6

AvWOW

Original post by ihaspotato

CORRECT! but but hooow?!??

I gave the explanation ^. I wrote in terms of phenolpthalein, but it applies to Thymol Blue (base)
Basically vertical region for weak acid strong base is around 7-10.5 and the pH range for Thymol Blue (base) is 8.0-9.6 (from the data booklet) which is WITHIN the range :smile:

so giving a clear colour change

Reply 7

ihaspotato

Original post by AvWOW

so giving a clear colour change

I'm sorry for making you write all that even after explaining. :frown:

Thank you for helping! :h:

Reply 8

Zahid~

ethanoic acid is a weak acid (all carboxilic acids are weak) , naoh is a strong base.
If you drew/imagined a ph curve, it would start of at at around three, the acid is weak so its not very acidic, so the verticle region of the ph curve will start high (like 5-6) and since youre adding a STRONG base, the vertical region of the ph curve will be at a high ph (like till 12) then it curves off again, So the indicator you use should have a change in colour between the ph 7-11.
Thymol blue is 1.2-2.8 so cant use
Bromo is 6-7.6 too low aswell
Thymol is 8-9.6 so maybe
Azaril is 10-12.0 which seems alittle to high. You want the ph range to be as close to the middle of the verticle region as possible

This is what the ph curve would look like:

So thymol is the best bet.

Note:- once you know all your ph ranges, and the ph curves (how the curve looks like if its a weak or strong acid, and the same for a base) AND once you know what is a weak/strong acid/base. You should be able to do these easily.

Reply 9

AvWOW

Original post by ihaspotato

I'm sorry for making you write all that even after explaining. :frown:

Thank you for helping! :h:

hahaha. you're welcome :smile:

Reply 10

ihaspotato

Original post by AvWOW

For strong acids it's commonly Phenolpthalein

Where can I find the list to what is strong/weak acid and what is strong/weak base? or do I have to work that out myself?

(edited 7 years ago)

Reply 11

Zahid~

http://www.chemistry.pomona.edu/chemistry/1alab/www/fall2006/powerptpresentations/5anions/acidbaset.htm
http://www.science.uwaterloo.ca/~cchieh/cact/c123/wkacids.html

just remember the ones you usually come across

Reply 12

ihaspotato

Original post by Zahid~

THANK YOU! I really needed this.

Don't know how to thank you more. Here's one from me.

Edit: Aww I can't give anymore reps for today. :/

(edited 7 years ago)

Reply 13

AvWOW

Original post by ihaspotato

Where can I find the list to what is strong/weak acid and what is strong/weak base? or do I have to work that out myself?

oops! I meant for strong BASES it's normally phenolpthalein :3 I didn't really have such a list, but weak acids tend to be things like carboxylic acids. You can compare the strength using the Ka values.
Strong tends to be more like H2SO4, HCl, HNO3 etc

Reply 14

ihaspotato

Original post by AvWOW

Data booklet
could you explain how Ka values can be used to compare strong/weak acid bases? eg. larger Ka value stronger the acid?

Reply 15

ihaspotato

Original post by ihaspotato

Data booklet
could you explain how Ka values can be used to compare strong/weak acid bases? eg. larger Ka value stronger the acid?

Found this. Posting it here if someone comes across this post asking the same question >>

https://www.khanacademy.org/science/organic-chemistry/organic-structures/acid-base-review/v/ka-and-acid-strength

still does not explain how to use Ka/pKa from the data booklet to for answering faster :s-smilie:

(edited 7 years ago)

Reply 16

AvWOW

Original post by ihaspotato

Data booklet
could you explain how Ka values can be used to compare strong/weak acid bases? eg. larger Ka value stronger the acid?

I am soooo sorry but I just saw your message :frown:

I really really hope you didn't suffer because I didn't check TSR :frown:

Again so so sorry. Hope you found the paper well? IAL or GCE?

Reply 17

ihaspotato

Original post by AvWOW

I am soooo sorry but I just saw your message :frown:

I really really hope you didn't suffer because I didn't check TSR :frown:

Again so so sorry. Hope you found the paper well? IAL or GCE?

No worries mate! I learn't it from a friend right before I entered the exam venue. :wink:

The paper (IAL) went kinda well... wouldn't say it was my best paper or the worst that I gave. Plus I don't even remember what the questions were or the answers that I gave - after I left the exam hall. I went to the chem discussion here and the replies on that thread are kind of scary since I wish I knew what they were talking about - making me feel like I gave all the wrong answers. :s-smilie:

But it's no use worrying about it now... Unit 5 Chem has got to go good no matter what!

(edited 7 years ago)

Reply 18

AvWOW

Original post by ihaspotato

No worries mate! I learn't it from a friend right before I entered the exam venue. :wink:

But it's no use worrying about it now... Unit 5 Chem has got to go good no matter what!

IAL too! I found it to be the easiest paper so far... Which makes me just a little afraid of unit 5.
But with a unit 6 like that, they really can't make it any harder