VCE Chemistry Question Thread

[Redoxify]

Thanks so much!

So how does the cathode lamp work? Is the light energy emitted as the electrons move to a lower energy level, hence emitting energy? Also if the same energy is required for the sample being analysed, how do we achieve this? Cheers!

How does it work?
A voltage of energy is applied to the sample in the cathode ray lamp, therefore the metal sample has it's valence electrons excited, then when they return to ground state they release light energy. So yeah you're right, when electrons return to ground state (no longer excited) energy in the form of light is released. The point of using the the analyte in the cathode ray lamp is so that we can assure that all the metal sample will absorb it, however not all of the light will be absorbed as that is dependant on the concentration of metal sample, hope that makes sense

[RazzMeTazz]

I have a few questions about NMR.

- Nucleons can spin in an 'up' or 'down' direction and in a nucleus when there is an odd number of nucleons the nucleus then displays an overall spin. Does this overall spin also have a general direction? So, if there was a C-13 isotope, and 6 of its nucleons had an overall 'up' spin and 7 of them had an overall 'down' spin, would the overall spin of the C-13 isotope be 'down'?

- Also would it be correct to say that only nuclei that are aligned with the external magnetic field can resonate by absorbing particular frequencies of radiowaves? ( as those aligned against the external magnetic field are of higher energy and so don't need  energy to resonate and become aligned with the external magnetic field?)

- Lastly, if the strength of the external magnetic field is kept constant, are the different chemical environments of the nuclei the only reason that they resonate at different radio wave frequencies to one another?

Any help would really be appreciated, I'm pretty confused about NMR :/

[grannysmith]

Can someone help me with Atomic Absorption Spectroscopy, I just don't get what it is and why it's used for?

Redoxify has already answered your question but oh well..

It's used to determine the concentration of an analyte in a solution. It exploits the fact that different metals have different electronic configurations and hence their respective electrons absorb different (specific) frequencies of light. As these electrons absorb light (energy), they are 'excited' and move to higher energy levels. This corresponds to absorbance. The great thing about this is that the amount of light absorbed is proportional to the concentration of the metal (cations/anions) in said solution.

However, this is only possible if we first know which frequencies of light will be absorbed by our metal of interest. The workaround is by setting up a cathode lamp which contains the metal of interest. These cathode lamps emit light at the exact frequency which will be able to be absorbed by ONLY the metal ions of interest in our sample. This makes sense because of how the cathode lamp is setup in the first place.

 

[cosine]

The recommended daily allowance of iron for people ove 11 is 18mg. What percentage of your daily needs does a 250mL glass of this drink provide?

I worked out there is 6425ug in 250mL of this drink, how do I do this question? Cheers.

[cosine]

If a 0.00ppm of ion concentration produced a 0.010 absorbance reading, provide an explanation of why this might have occurred? Also what is ppm? and how can we convert it to other readings? Cheers!

[grannysmith]

The recommended daily allowance of iron for people ove 11 is 18mg. What percentage of your daily needs does a 250mL glass of this drink provide?

I worked out there is 6425ug in 250mL of this drink, how do I do this question? Cheers.

Convert 6425 micrograms into grams (10^-6) and divide by 18 x 10^-3. Multiply by 100 to get a percentage.

If a 0.00ppm of ion concentration produced a 0.010 absorbance reading, provide an explanation of why this might have occurred? Also what is ppm? and how can we convert it to other readings? Cheers!

There could have been impurities in the solution which absorbed some of the light.
ppm is a measure of concentration, generally measured as mg/L.

[Redoxify]

If a 0.00ppm of ion concentration produced a 0.010 absorbance reading, provide an explanation of why this might have occurred? Also what is ppm? and how can we convert it to other readings? Cheers!

Very simply, to change the micro grams to grams, divide by 1000, much more easy to work with.
ps ppm stands for parts per million or (milligrams/litre)

[cosine]

Convert 6425 micrograms into grams (10^-6) and divide by 18 x 10^-3. Multiply by 100 to get a percentage.
There could have been impurities in the solution which absorbed some of the light.
ppm is a measure of concentration, generally measured as mg/L.

How can impurities absorb some of the light if the energy required for the electrons to move to a higher energy level is exactly the same as the enegy emitted?

[Redoxify]

How can impurities absorb some of the light if the energy required for the electrons to move to a higher energy level is exactly the same as the enegy emitted?

you're getting different techniques confused, this form of uv visible shoots a particular wavelength of energy, and can get absorbed by anything in the solution (impurities) therefore we get that small reading of absorbance

[lzxnl]

I have a few questions about NMR.

- Nucleons can spin in an 'up' or 'down' direction and in a nucleus when there is an odd number of nucleons the nucleus then displays an overall spin. Does this overall spin also have a general direction? So, if there was a C-13 isotope, and 6 of its nucleons had an overall 'up' spin and 7 of them had an overall 'down' spin, would the overall spin of the C-13 isotope be 'down'?

- Also would it be correct to say that only nuclei that are aligned with the external magnetic field can resonate by absorbing particular frequencies of radiowaves? ( as those aligned against the external magnetic field are of higher energy and so don't need  energy to resonate and become aligned with the external magnetic field?)

- Lastly, if the strength of the external magnetic field is kept constant, are the different chemical environments of the nuclei the only reason that they resonate at different radio wave frequencies to one another?

Any help would really be appreciated, I'm pretty confused about NMR :/

NMR is taught pretty awfully at VCE level tbh.
1. Yes. That's how NMR works; as the overall spin is non-zero, the nucleus as a whole can interact with the external magnetic field. It's a little like asking what's going to happen if you put an electrically neutral object in an electric field. Not much is going to happen there; you'd need some charge somewhere.

2. Yes. The nuclei aligned against and the nuclei aligned with the magnetic field would be in equilibrium (thermodynamically it's actually not stable to have all the nuclei aligned with the magnetic field for reasons I won't go into here; wait until second or third year statistical mechanics for that)

3. Yes. That's the essence of NMR. As different hydrogens (tend to) absorb at different frequencies depending on their chemical environment, NMR allows probing of the number of chemical environments in a molecule, which can be remarkably informing at times.
For instance, if you're given the molecular formula C6H3Cl3 and you're told there's only one H1 NMR peak, that's enough for you to work out a structure already

[KingDrogba]

How is ethanoic anhydride both a reactant and a solvent during the preparation of Aspirin when it reacts with Salicylic Acid in the presence of Sulfuric acid?

[jessss0407]

Hi everyone,

Can someone please explain molecules are always in a liquid state for esterification, also, how can we determine the states for chloroalkanes (e.g. chlorobutane) and other molecules with functional groups?

Thanks

[lzxnl]

How is ethanoic anhydride both a reactant and a solvent during the preparation of Aspirin when it reacts with Salicylic Acid in the presence of Sulfuric acid?

It's a liquid and it's more reactive than ethanoic acid (for reasons VCAA doesn't care about). It's not part of the course, so you don't need to worry about it.


Does that answer sound unsatisfying? If so, blame VCE's lack of depth

[cosine]

If the percentage of a compound (eg. Phosphate) by mass in the fertiliser is less than what is specified by the manufacturer, lost two reasons what can be accounted for this.

The technique used is colorimeter. Thanks

[alchemy]

If the percentage of a compound (eg. Phosphate) by mass in the fertiliser is less than what is specified by the manufacturer, lost two reasons what can be accounted for this.

The technique used is colorimeter. Thanks

Well, manufacturers have much more sophisticated equipment than the stuff you have at school and therefore can make more accurate measurements. So the chances are if there is a difference with your calculated amount to the manufacturer specifications, the mistake is on your behalf.
Also, as was said earlier in the page, the presence of other substances in the mixture (impurities) may have affected the absorbance reading if using colorimetry, resulting in a different calculated % mass...