Chem HSC help (1 Viewer)

[deswa1]

Hey guys,

Can you help me with these questions (from 2010 HSC)? Can you explain the answers:

7 Equal volumes of four 0.1 mol L acids were titrated with the same sodium hydroxide solution.
Which one requires the greatest volume of base to change the colour of the indicator?
(A) Citric acid
(B) Acetic acid
(C) Sulfuric acid
(D) Hydrochloric acid

Q14- I've taken a screenshot of this Q and attached it.

Thanks heaps

 

[Aesytic]

the answer is A because citric acid is triprotic --> C3H5O(COOH)3 ---> 3H+ + C3H5O(COO)3 -3
if you were to write out neutralisation reactions for each of those 4 acids, you would see that because citric acid dissociates into 3 hydrogen ions, the mole ratio of NaOH to citric acid would be 3:1, hence more moles of NaOH would be needed (the moles of the 4 different acids would be the same in this situation, since their volumes and concentrations are the same) with citric acid than with any of the other options. since volume = moles/concentration, and the concentration of NaOH is constant, more moles would mean a greater volume, and hence the answer would be A.

i'm not really sure about 14 though...

 

[iSplicer]

Hey guys,

Can you help me with these questions (from 2010 HSC)? Can you explain the answers:

7 Equal volumes of four 0.1 mol L acids were titrated with the same sodium hydroxide solution.
Which one requires the greatest volume of base to change the colour of the indicator?
(A) Citric acid
(B) Acetic acid
(C) Sulfuric acid
(D) Hydrochloric acid

Q14- I've taken a screenshot of this Q and attached it.

Thanks heaps

Not sure if the screenshot is of the correct question...? In any case, the answer is 31*C. Being an ester, it lacks the polar hydroxyl/carboxyl functional groups and thus its ability to form hydrogen bonds is all but cancelled, leading to a lessened degree of aggregate intermolecular attraction. It therefore has to be lower in BP than all the other listed compounds.

And Aesytic's answer is spot on. Make sure you include that equation with states in your exam answer to ensure full marks.

 

[deswa1]

Oh yes, thanks heaps to both of you. Would rep but yeah (though I think that jokes old now ).

If either of you (or anyone else) has time, can you help with two other things:
- Firstly, with titration, you have to decide which indicator to use. Let's say if you titrate NaOH with HCl. The solution at the equivalence point will be neutral (pH=7). Does this mean we need an indicator that changes at 7 or something around there? For example, can we use phenolphthalein even though that has a lower 'bound' (don't know the technical term) of 8.3?

Also, I got an answer for this but I want to confirm:
Question: Using 0.100 mol L NaOH a student titrated 25.0 mL of a 0.100 mol L weak monoprotic acid and separately titrated 25.0 mL of a 0.100 mol L strong monoprotic acid. Which statement about the volume of base required to reach the equivalence point is correct?
A- The weak acid will require the same volume of base as the strong acid.
B- The weak acid will require a larger volume of base than the strong acid
C- The weak acid will require a smaller volume of base than the strong acid
D- The volume of base required will depend on the molar mass of the acid use

I'm not very good at titration .

Love you guys

 

[nightweaver066]

Because it's a weak acid, it will not completely ionise compared to the strong monoprotic acid.

So there will be more H+ ions in the strong monoprotic acid solution than in the weak acid solution.

This means it would require a smaller volume of base (because there will be less OH- ions) to reach the equivalence point.

So the answer is C.

 

[iSplicer]

Because it's a weak acid, it will not completely ionise compared to the strong monoprotic acid.

So there will be more H+ ions in the strong monoprotic acid solution than in the weak acid solution.

This means it would require a smaller volume of base (because there will be less OH- ions) to reach the equivalence point.

So the answer is C.

Wrong. You fell for this CLASSIC trick that they love asking, haha.

It's irrelevant how strong/weak an acid is, because during titration/neutralisation, the equilibrium point of the weak acid's dissociation will be shifted along to compensate for the neutralisation. Yes, in a clean solution, only a small portion of a weak acid's hydrogen ions will have dissociated, but if I were to neutralise this by adding a base, hydrogen ions will virtually be 'taken out' of the solution (to form water), and by Le Chatelier's principle the weak acid's equilibrium position would shift to the right (producing more H+ ions) to compensate as necessary.

The answer is A.

 

[nightweaver066]

No you can't use phenolphthalein, it changes too late/early (depending which way you titrate it).

Ideally, you want an indicator that changes colour right at the equivalence point, where one of the bounds is exactly where the equivalence point will be.

Since we can't do that, we pick indicators that change colours as close to the equivalence point as possible, so we use bromothymol blue which would change colour at 6.0 or 7.6 rather than 8.3 with phenolphthalein, which is much off.

 

[nightweaver066]

Wrong. You fell for this CLASSIC trick that they love asking, haha.

It's irrelevant how strong/weak an acid is, because during titration/neutralisation, the equilibrium point of the weak acid's dissociation will be shifted along to compensate for the neutralisation. Yes, in a clean solution, only a small portion of a weak acid's hydrogen ions will have dissociated, but if I were to neutralise this by adding a base, hydrogen ions will virtually be 'taken out' of the solution (to form water), and by Le Chatelier's principle the weak acid's equilibrium position would shift to the right (producing more H+ ions) to compensate as necessary.

The answer is A.

lol thanks, funny because i got this same thing wrong a few weeks ago ==

Now this is definitely drilled in to me haha

 

[iSplicer]

No you can't use phenolphthalein, it changes too late/early (depending which way you titrate it).

Ideally, you want an indicator that changes colour right at the equivalence point, where one of the bounds is exactly where the equivalence point will be.

Since we can't do that, we pick indicators that change colours as close to the equivalence point as possible, so we use bromothymol blue which would change colour at 6.0 or 7.6 rather than 8.3 with phenolphthalein, which is much off.

Wrong again, you definitely can use phenolphthalein in a strong acid-strong base titration. The inflexion point lies on a VERY steep pseudo-vertical line on the titration curve, resulting in an equivalence point that still lies well within this region.

 

[iSplicer]

lol thanks, funny because i got this same thing wrong a few weeks ago ==

Now this is definitely drilled in to me haha

No probs =]. How awesome would it be if it popped up in your trials, haha =)

 

[nightweaver066]

Wrong again, you definitely can use phenolphthalein in a strong acid-strong base titration. The inflexion point lies on a VERY steep pseudo-vertical line on the titration curve, resulting in an equivalence point that still lies well within this region.

I should have phrased myself differently.

Yes, you can use it, but wouldn't bromothymol blue be more correct?

 

[someth1ng]

I should have phrased myself differently.

Yes, you can use it, but wouldn't bromothymol blue be more correct?

No, phenolphthalein is probably the best.

 

[golgo13]

Actually i agree is isplicer, cause the and it's A, although logically you think it should be less, titration is all about the degree of acidity. So if i say had a weak diprotic acid, i would need twice the amount irrespective of the strength. The strengths help pick the indicator, in this case the equilibrium would be at the basic end so you would use phenolphthalein (Strong base vs weak acid), use methly orange (Strong acid vs weak base), use bromo for the other 2

 

[BL3H]

No probs =]. How awesome would it be if it popped up in your trials, haha =)

It was in the HSC last year (could have been trials) so I'd doubt it.