what's the appropriate indicator--Ochem

[PinkDDS]

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An acid has a pka of 1.8x 10^-5. The appropraite indicator to use when titrating a 0.1 M solution of this acid with 0.1 M potassium hydroxide is:

A. methyl red (pH range 4.4 to 6.2)
B. Malachite green (pH range 0.2 to 1.8)
C. Congo red (pH rang 3 to 5)
D. Phenolphthalin (pH range 8.2 to 10)

Can anyone explain how this problem is done?
Thanks

 

[DDSguyLA]

hey..
doesnt pH equal pKa in this case.
then just find pH and match the range.
the colors might be there to through us off.

 

[IDoWhatIWant]

Correct me if I am wrong, but this is a titration of a weak acid with a strong base. The pH indicator used should have a basic range. I would go for D. *crossing fingers*

 

[PinkDDS]

hey..
doesnt pH equal pKa in this case.
then just find pH and match the range.
the colors might be there to through us off.

Since this is a weak acid being titrated by a strong base, at the equivalence point the solution is defined by Kb. So we need to get the pOH first. From pOH, pH can be calculated, but I don't get the right answer for some reason 😕

 

[PinkDDS]

Correct me if I am wrong, but this is a titration of a weak acid with a strong base. The pH indicator used should have a basic range. I would go for D. *crossing fingers*

You are right🙂! The answer is D. Do you know how to calculated the pH?

(the answer is pH=8.72)

 

[DDSguyLA]

yeah... i suck at pH acid/base.
its nothing new.

 

[PinkDDS]

yeah... i suck at pH acid/base.
its nothing new.

Same here 😳

 

[IDoWhatIWant]

You are right🙂! The answer is D. Do you know how to calculated the pH?

(the answer is pH=8.72)

Well, it's not necessary to calculate the pH. The DAT is a very logical test. What you do need to remember are the titration curves. So here are different scenarios:

Strong acid/Strong base (and vice versa):
pH indicator used should be around the equivalence point: pH 7.

Strong acid/Weak Base:
because of the presence of a strong acid (doesn't matter which is titrating which), the indicator used should be around pH of less than 7.

Strong base/weak acid:
Following the same logic, pH indicator --> basic range.

I hope this helps.

 

[PinkDDS]

Yes, it did help. Thank you🙂

 

[doc toothache]

An acid has a pka of 1.8x 10^-5. The appropraite indicator to use when titrating a 0.1 M solution of this acid with 0.1 M potassium hydroxide is:
A. methyl red (pH range 4.4 to 6.2)
B. Malachite green (pH range 0.2 to 1.8)
C. Congo red (pH rang 3 to 5)
D. Phenolphthalin (pH range 8.2 to 10)

Phenolphthalein is the indicator generally used in acid-base titrations. Since we are dealing with dilute acidic solutions, the 3 other choice would not serve us too well, especially in this case where the pH of the acid in question is <3.