Soccerboi's questions thread

[Mao]

1. The end point and the equivalence point are different things. The equivalence point is the point at which reaction has actually completed. The end point, however, is the visual cue indicating that the equivalence point is achieved (e.g. indicator changing colour). Thus, the end point always occurs after the equivalence point; the end point is the result of adding, say, a little more acid than at equivalence, which thus alters the pH such that the indicator will change colour. Generally, the difference between the end point and equivalence point is very small because of the large change in pH generated by the addition of a single drop of whatever you're titrating.

That's incorrect. There is no physical coupling between the end point and the equivalence point.

Where the end point is depends on the choice of indicator. The indicator changes colour at its characteristic pH range, dependent on the chemical structure of the indicator. Specifically, this is strongly related to the pKa of the indicator (concept above VCE). Anyhow, the data booklet gives the pH range of end points of the indicators.

Crucially, we must choose an appropriate indicator that has an end point near where the equivalence point is. The fact that we generally say end point = equivalence point is because we choose the indicator which has an end point = equivalence point for that particular experiment.

[soccerboi]

A few more questions:
1. Why is it that the non metal component of the sample in a flame test responsible for the flame colour?

2. Is a glycosidic link the O by itself or is it the C-O-C ?

3. Do we consider the sig figs of molar masses? e.g we used the molar mass of Na 23.0(3 sig figs) but in the stem of the question, the least we used was 4 sig figs, so do we leave our final answer as 4 sig figs or 3 because of the molar mass from the data booklet?

[Aurelian]

That's incorrect. There is no physical coupling between the end point and the equivalence point.

Eh yeah, sorry; I was explaining from a perspective which assumed an "appropriate" indicator had already been chosen. I should have clarified this. Thank you for pointing that out =)

A few more questions:
1. Why is it that the non metal component of the sample in a flame test responsible for the flame colour?

2. Is a glycosidic link the O by itself or is it the C-O-C ?

3. Do we consider the sig figs of molar masses? e.g we used the molar mass of Na 23.0(3 sig figs) but in the stem of the question, the least we used was 4 sig figs, so do we leave our final answer as 4 sig figs or 3 because of the molar mass from the data booklet?

1. Not sure of a thorough answer, but I imagine it would have to do with the behaviour of the valence electrons in terms of energy levels. I wouldn't worry about this...

2. Just the O I believe; this is the actual "link".

3. Yes, molar masses count towards significant figures.

[soccerboi]

Thanks Aurelian

[dinosaur93]

1. Which substance, when dissolved in water to give a 0.1 mol dm^-1 solution, has the lowest pH?

A. HCl

B. NaCl

C. NH₃

D. H₂SO₄


2. Consider the reaction

MnO₂ + 4HCl Cl₂ + H₂O + MnCl₂

The atoms whose oxideation number change during this reaction are:

A. Mn

B. Mn and HCl

C. Mn, Cl, and O

D. Mn, Cl, O and H


3. Which of the following statement in regard to the IR Spectra of important biochemicals is incorrect.

A. Oleic Acid - absorption at the wavenumber of 1750cm-1
B. Glycerol - absorption at the wavenumber of 3300cm-1
C. Glycerine - absorption at the wavenumber of 3500cm-1 and 3200cm-1
D. Steric Acid - absorption at the wavenumber of 1610cm-1 and 1700cm-1


4. 1.245g of impure silver reacted with concentrated nitric acid, which is diluted to make 200.0mL of solution. A 20.00mL aliquot taken and reacted with excess sodium chloride. The resulting solid is filtered, rinsed and dried, and has a mas of 0.165g. The purity of the silver is ?


5. A saturated fat is hydrolised, Both glygerol and fatty acid molecules are produced in the reaction, Which one of the following is a possible formula for the acid?

A. C16H30O2

B. C18H3602

C. C20H38O2

D. C24H44O2

How did you derived in getting this answer, what if it is a poly unsaturated or mono saturated fatty acid that was produced?


6. Which statement about coompound I na d compound II is not true?

Compound I = CH₃CH₂OH

Compound II = CH₃COCH₃

A. the mass spectrum of both compounds show a peak at a mass-to-charge ratio of 58.

B. The low resolution Proton NMR spectrum in Compound I has 3 peaks and that of compound II has 2 peaks...

C. The 13-C NMR Spectrum of compound I has 3 peaks and that of compound II has 2 peaks

D. The fingerprint region of the IR Spectra will be different for both compound.



7. Peptide are formed via the pathway of

A. addition polymerisation between an alkanol and a carboxylic acid.

B. Condensation polymerisation between an alkanol and carboxylic acid

C. Addition polymerisation between an alkanol and an amine.

D. condensation polymerisation between an carboxylic acid and an amine..



8. A stomach enzyme is heated and denatured, The bonds which holds an enzyme together include

I. dispersion forces
II. dipole-dipole bonds
III. Hydrogen bonds
IV. Covalent bonds

When the enzyme is heated from 25-90 degrees celcius, bonds affected include:

A. I and IV

B. I and III

C. I, II, III and IV

D. II and II only


9. What are the compound/s which could be used as a primary standard to standardise a solution of sodium hydroxide.

A. Sodium Borate

B. Anhydrous Sodium Carbonate

C. Hydrochloric acid

D. Sulfuric Acid

E. Hydrated Oxalic Acid

F. Calcium Hydroxide.



(mod edit: just fixed the latex for you )

[ecvkcuf]

elvin what questions are those from?

because I don't think they are VCAA standard?

[pi]

I'll just do the ones I remember (these are VCAA standard btw):

1. D (not sure on this one)

2. B

5. B

6. A

7. D

8. C


I hope those are right, I'm pretty rusty :S

[soccerboi]

VCAA 2002. Q1b)
A pure substance was suspected of being the high molecular weight anabolic steroid Stanozolol. A sample
of this substance was dissolved in a suitable pure solvent and injected into a gas chromatograph. How
many peaks would you expect to see in the resulting chromatogram? Justify your result.

It says "high molecular weight" and in heinenman textbook it says GC usually is used for substances less than 300 g mol^-1, so how do we know the molecular mass of stanozolol if the molecular formula is not given? Like since its high molecular weight, what if its too high to be used for GC. Also how would u know how many components it is made of and thus how many peaks it produces? Also whats the solvent and why does that also produce a peak?

[charmanderp]

We assume that it's not too high, given that otherwise the question wouldn't ask it.

If you inject a sample of a pure substance this would produce one peak. Then we have an additional peak from the solvent, which is also pure. So two peaks overall. The key word in the question is pure.

The solvent is used to dissolve the substance. We can't exactly put a solid through a GC, could we? So our mixture is essentially made up of two components (solvent and Stanozolol), each of which would have relative affinities for the stationary phase and each of which would be picked up by the detector after exiting the machine. Hence both would produce individual peaks.

[soccerboi]

Thanks a lot, this question has been bothering me for a while and your explanation has cleared everything up 

[soccerboi]

1. The crocin in a dried 4.875 g sample of saffron is extracted into solution. The solution is filtered and made up to 100 mL in a volumetric flask. 10.0 mL of this solution is then further diluted to 250 mL. The absorbance of the diluted solution at the same wavelength as that used in the calibration was found to be 0.22.

i. Calculate the crocin content of the diluted solution, in g L–1.
0.1920gL^-1 (This was just from the graph)

ii. Calculate the percentage by mass of crocin in the saffron sample.
9.85% <------ How do i get this???



2. Write a balanced chemical equation to show the combustion of ethanol for the release of energy.

Solution
C2H5OH(aq) + 2O2(g) → 2CO2(g) + 3H2O(g)

I thought that it isn't possible to combust a compound in  (aq) ?

[Phy124]

1. The crocin in a dried 4.875 g sample of saffron is extracted into solution. The solution is filtered and made up to 100 mL in a volumetric flask. 10.0 mL of this solution is then further diluted to 250 mL. The absorbance of the diluted solution at the same wavelength as that used in the calibration was found to be 0.22.

i. Calculate the crocin content of the diluted solution, in g L–1.
0.1920gL^-1 (This was just from the graph)

ii. Calculate the percentage by mass of crocin in the saffron sample.
9.85% <------ How do i get this???

You have a content of 0.1920 g/L in the diluted solution

You have to multiply this by 25 (the dilution factor - you took 10mL of the original 100mL [same C] but then diluted to 250mL so 1/25 original) to get 4.8g/L

So for every litre of solution we have 4.8 grams of crocin, but we only have 100mL of solution, so we have 0.48 grams of crocin.

0.48 grams crocin in the 4.875 grams sample of saffron.

%%

[soccerboi]

Would the compound below have 3 or 4 H environments? I'm not sure about the OH on the left end. Is that 1 environment or is that part of the CH2OH environment?

[Phy124]

Would the compound below have 3 or 4 H environments? I'm not sure about the OH on the left end. Is that 1 environment or is that part of the CH2OH environment?

Four.

On the left, the two Hydrogen atoms attached to the Carbon atom are one environment, the Hydrogen atom attached the Oxygen atom is also one environment. Then moving to the right the two Hydrogen atoms attached to the Carbon atom are one environment and the Hydrogen atom attached the Oxygen atom is one environment, totaling four.

Concentrating on your specific question, think about the nearest adjacent atom, for one it is an Oxygen atom and for another it is a Carbon atom, thus they can't be the same environment.

[soccerboi]

Thanks alot