A2 OCR Chemistry B F334 - June 2015

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Does anyone know why you have to refer to the Cr being a stronger reducing agent than the Fe. Why can't you say the Cr3+ ions are a stronger reducing agent than the Fe2+ ions?

Just a guess but in eii) it is the Cr that is being oxidised by the hydrogen half cell and the same would happen to Fe because it also has a negative E cell value. This means it is the metals in solid state that are losing electrons so are reducing agents. That means if thinking of half equations, the electrons are on the same side as the ions so it is the ions that would be reduced so act as the oxidising agent.

I don't understand why my answer was wrong. I've drawn what I did on the left of the attached picture. I thought since it's yellow I thought it would reflect the yellow wavelengths and absorb everything else, but this is wrong? (the correct one is on right hand side of the attached pic)

Is the x-axis wavelength or frequency on the right? I think u did frequency on the left. And its cos it tells you that yellow is made from colours of different wavelengths.

Ah yeah it's wavelength on the right, I didn't realise that I'd don't frequency, thanks. How come it doesn't start reflecting at the very end of the graph when it's at the red wavelength?

Ah yeah it's wavelength on the right, I didn't realise that I'd don't frequency, thanks. How come it doesn't start reflecting at the very end of the graph when it's at the red wavelength?

im confused. is there a reflectance spec with frequency then? or its always wavelength? also you did ROYGBIV in the wrong direction i swear its VIGBYOR along the X axis, red has the highest wavelength not the lowest

I think of reflectance spectrums as the wavelengths highly reflected being what you see as the colour. The 93% red shows a high transmittance/reflectance of red to make this shade of yellow and on a graph of reflectance against wavelength, the line you draw starts off high on the far right which is the area where the red range is. 90% green shows that as you move leftwards there is no real change in reflectance from red range to green range. 0% blue shows that the far left on the graph, reflectance would be at its lowest so there is a dip in reflectance and then it just remains roughly constant as shown by the graph on the right.

Hi everyone, hope revision is going well
I've come across this questions about high resolution mass spectrometry and and how the mass:charge ratio of the molcular ion peak can be used to determine the molecular formula

Can someone explain this to me, please!
It's something that I'm not familiar with and seemed to have completely missed whilst learning the content
Thanks!

Hi, I haven't gone over this in detail myself but from the top of my head, the m/z ratio is on the x axis and using this information you can work out the Mr of whatever fragments you're looking at. (M/Z ratio is essentially the same as the Mr and is caused by ions with a positive charge). So for example if there's a peak at 15, this could be the fragment CH3+ etc. Similarly, the M+ peak = the molecular formula of the WHOLE ion and therefore the largest ion, so if there's a peak at 80 then that would be the molecular mass, after which you use information from the question or whatever else they give you to deduce what elements that 80 could consist of.

Do bear in mind that if there's a peak at 81 or so that could be the Carbon-13 isotope peak or any other isotope peaks (such as M+2 for Cl) depending on the molecule being analysed, but generally it would be much smaller and obvious that it is an isotope peak.

High resolution mass spectrometry just gives you the molecular mass to 4 decimal places which makes it more accurate when you're trying to differentiate between two molecules that have a similar Mr in a low resolution spectrum.

Maybe I'm just repeating what you already know lol but I think that's all there is to it!

Wow thank you so much for going into so much detail!
So is the mass:charge ratio the same as the M/Z ratio?
I had never come across high resolution mass spec. whilst revising so thanks for going over that!

Why isn't the NH2 in the amide protonated when drawing this zwitterion?

This is because the zwitterion has a neutral charge so only one NH2 group can be protanated to cancel out the charge form COO-

However, if you added acid to the zwitterion then both NH2 groups would be protonated as its no longer at the isoelectric point so doesnt exist as a zwitterion no longer

Hope that helps !