TrueTears question thread

[TrueTears]

Also this one,

The amount of Vitamin C in fruit juice can be determined by titration with standard 0.0100M iodine solution.



If the maximum concentration of vitamin C is likely to be , describe how you would perform the analysis. You should mention a) the volume of fruit juice used. b) the maximum titre of iodine you would expect to obtain.

Many thanks.

[/0]

1. Fill the burette with iodine solution. Record intial volume.
2. Pipette 20 mL of fruit juice into a conical flask.
3. Add 2-3 drops of starch indicator to fruit juice.
4. Titrate until endpoint is reached, recording final volume.
5. Repeat steps 1-4 until three concordant tires within 0.10mL of each other are obtained.

Since the concentration of vitamin C is , in about 20mL of fruit juice (you could also have other volumes of fruit juice, but I choose 20mL) there is 0.01g of vitamin C.







So the expected titre is about 5.86mL

[TrueTears]

thanks heaps ST

[TrueTears]

The following materials are commonly used in chromatography: water, ethanol, paraffin wax, glass sheets, paper strips, powered alumina, nitrogen gas and hexane. Choose a suitable stationary and mobile phase from the list to be used in the analysis of amino acids by thin-layer chromatography.

thanks again XD

[TrueTears]

also on a chromatograph, does low peaks mean the component is more soluble in the stationary phase? And the higher the peaks are means they are less soluble in the stationary phase?

[TrueTears]

Iron is essential to our health. To determine the iron content in a flavoured milk drink, a 5 mL sample was diluted to 50mL. The absorbance of the diluted solution and several standard solutions was measured using atomic absorbance spectroscopy.
Suggest why the sample of the drink was diluted to measure its absorbance.

[kj_]

The following materials are commonly used in chromatography: water, ethanol, paraffin wax, glass sheets, paper strips, powered alumina, nitrogen gas and hexane. Choose a suitable stationary and mobile phase from the list to be used in the analysis of amino acids by thin-layer chromatography.

thanks again XD

Stationary phase: powered alumina. This is a essential feature of thin-layer chromatography. A very fine powdered adsorbent material is placed on the glass / plastic / sheet of metal. Out of all the materials you have listed, powered alumina is the suggested material.
Mobile phase:water or ethanol, however, I'm presuming amino acids may react with ethanol, and thus water should be used. The mobile phase should be unreactive with the substance under analysis.

also on a chromatograph, does low peaks mean the component is more soluble in the stationary phase? And the higher the peaks are means they are less soluble in the stationary phase?

Generally on a chromatograph, the vertical axis is the measure of abundance. The higher percentage of component within the sample, the higher the peak will be. Unless we're talking about different graphs, the height of the peaks does not refer to the solubility of the component.

Iron is essential to our health. To determine the iron content in a flavoured milk drink, a 5 mL sample was diluted to 50mL. The absorbance of the diluted solution and several standard solutions was measured using atomic absorbance spectroscopy.
Suggest why the sample of the drink was diluted to measure its absorbance.

The calibration graph produced from an AAS is linear for small concentrations only (for VCE purposes). Thus, we have to dilute the sample to decrease the concentration of iron present, so that a linear calibration graph can be obtained.

[TrueTears]

ahh thank you so much kj_ that really clears it up. XD

[TrueTears]

Also just this NMR question, im kind of confused with NMR just wondering if someone could explain through how they would do Q 18, include as much detail as you can XD thanks so much.

Here is Q 18


Here is the required "Table 7.9"


Thanks again.

[kj_]

a) How many different environments are the hydrogen atoms in? (Hydrogen is the same as a proton, if you were confused by that.) From the semi-structural formula, we see that H is in: CH3on the left, a CH2 connected to a COO and CH3, and a CH3 connected to a CH2. Thus, we have 3 different environments.
Also, you can look on the NMR spectrum and recognise 3 sets of results = three different environments.

b) From table 7.9, you want to find the chemical shift of the CH3 to the left, a CH2 connected to a COO and CH3, and a CH3 connected to a CH2.
For the CH3 to the left, CH3 -COO, I cannot find on that table, but the VCAA data book tells me that this value is 2.0ppm.
For the CH2 in the middle, O-CH2-R is a shift of 3.3 - 4.3ppm.
For the CH3 at the right, R-CH3 is a shift of 0.7 - 1.6ppm.

c) From the graph, note the ratio of A:B:C is 2:3:3. Thus, for every 3 protons of B and C, you have 2 A's

d)i. Splitting is a result of neighbouring hydrogens which are not "shielded off" (For example, splitting is "shielded" off by oxygen molecules.).
Peak A is a quartet as peak A refers to the CH2 in the middle. We count three (3) neighbouring Hydrogen molecules (appearing on the CH3 at the right.) Using the n+1 rule, where n is the number of neighbouring protons, have the peak splitting into 4.
ii. Peak B is single as this refers to the CH3 to the left of the semi-structural diagram. We notice that immediately neighbouring this Hydrogen atom is only a COO group - no H's are available to cause splitting.
iii. Referring to the CH3 group at the right, and the same explanation to part i.

e) Done that in part d.

f) How many peaks refers to the amount of different environments for the C compound. If we count, we can clearly see 4 different environments. (Try this yourself

Note: Re-read the theory behind these few chapters and make sure you understand them, and these questions will start to become easier

[TrueTears]

thanks very much kj_

[TrueTears]

also..
2. if you have to name , does that mean the Cl is on the first C of the whole chain, so this hydrocarbon would be called 1-chloropentane?

3. Name all the structural isomers of . Is there a systematic way to approach this? Or do you just keep creating different structures until you have created every possibility?
I got 4 different isomers: pent-2-ene, 3-methylbut-1-ene, 2-methylbut-2-ene, 2-methybut-1-ene. Am i missing any?

4. Do you name this 3-methylbut-2-ene or 2-methylbut-2-ene? Do you always number as close as possible to the double bond and the methyl group?


5. Butane's molecular mass is 58, boiling point is -0.5 deg Celsius
propan-1-ol's  molecular mass is 60, boiling point is 97 deg Celsius
chloroethane's molecular mass is 65, boiling point is 12.5 deg Celsius.
Even though these compounds Molar Mass is around the same why do they differ so much in boiling temperature?

I know the basic answer which is because they are in different homologous series, but how do you explain propan-1-ol's high boiling temp compared to butane having a much lower boiling temperature?

thanks again XD

[mark_alec]

1. 2-methylbutane. If you draw out the structure, you will see that there are two methyl groups attached to the one carbon.
2. You cannot determine which carbon the chlorine is on, chloropentane is the best you can do.
3. The way to do it is to draw all possible combinations, you have missed some, such as 1-pentene.
4. Since the double bond is on the second carbon (from whichever end), you want to minimise the number that the methyl group has, so in this case: 2-methyl-2-butene
5. Think about intermolecular bonding, dispersion vs polar vs hydrogen.

[TrueTears]

thank you very much mark_alec, yeah 1-pentene was part of the question haha i just didn't put that in this thread XD. So in VCE chem 3/4 we don't have to know cis, trans isomerism? Or optical isomerism?

[polky]

thank you very much mark_alec, yeah 1-pentene was part of the question haha i just didn't put that in this thread XD. So in VCE chem 3/4 we don't have to know cis, trans isomerism? Or optical isomerism?

Nope.  Those are geometric isomers (iirc) and it's not required knowledge for VCE chem!