Got a TSFX ad for mid year exams and they had a few qs which i have no idea about...
1) A sample of MgCl2 is placed in the flame of a Bunsen burner. The component of the substance which is responsible for the production of a flame colour is Mg2+. True/False and why?
2) Would different infra-red and proton NMR spectra be obtained for pure organic liquids as opposed to samples dissolved in water? What differences would you observe and why?
3) Do the isomers of a compound produce different IR spectra or would the spectra be the same? Does the same pattern apply to the spectra produced in NMR spectroscopy?
4) Chlorine has two isotopes. 35Cl and 37Cl and therefore, the mass spectrum of chlorine gas will show two peaks? Why isnt this corrent?
5) If a mixture of 4 low molecular weight organic compounds are resolved using GLC, 4 peaks would be obtained. Is this answer correct? Why is this incorrect?
6) Weak acids require a greater volume of base as compared to the same volume and concentration of a strong acid to reach the equivalenc point of a reaction. IS this true or false and why?
This is really worrying me as I really have no idea as to most of these and mid year exams are so close....
These are DESIGNED TO SCARE YOU. So don't worry. Many of them are not really central to the course...
1. False. Mg is responsible - in a flame, the metal forms a solid, then the solid crystals are dissociated to produce individual ground state atoms, whose electrons are then excited into higher energy levels.
2. Yes, different IR and NMR spectra would be obtained, as the O-H bonds in water will absorb IR radiation, and the H atoms in water will produce a peak on the proton NMR spectrum. Thus we need another solvent e.g. deuterium oxide.
3. Isomers will produce different IR spectra because they have different bonding arrangements. The NMR spectra will also be different.
4. In Mass spec, occasionally, ions will end up with a 2+ charge instead of a 1+ charge, causing them to have a different mass/charge ratio. which means there will likely be 4 peaks, with others potentially in addition.
5. Likely, they would overlap, so 4 distinct peaks may not be seen. GLC is particularly useful for high molecular weight compounds, and is able to differentiate between them as they have stronger dispersion forces than low molecular weight compounds.
6. False. the strength of an acid refers to how much it will dissociate when dissolved in water, not the ratio in which it will react with base.
Hope that helps
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