Dekoyl's Questions

[Mao]

The monochlorinated hydrocarbon is the most abundant.

Remember that this is a substitution reaction, i.e. only one of the Cl2 attaches onto the alkane. Hence the order of abundance is:

HCl > monochlorinated > dichlorinated (2 Cl2 reacted) > trichlorinated (3 Cl2 reacted) > tetrachlorinated > etc

where the concentration of HCl is the 1 * monochlorinated + 2 * dichlorinated + 3 * trichlorinated + etc...

[dekoyl]

A 2L sample of gaseous hydrocarbon is burnt in excess oxygen. The only products of the reaction are 8L of  CO2(g) and 10L of H2O(g) all at 100C and 1 atm pressure. What is the formula of the hydrocarbon?

I don't recall anything when looking at the answers. =(

Thanks

[Mao]

remember that in gases, the volume is proportional to the number of moles.

X --(burnt in oxygen)--> 4CO2 + 5H2O

ignoring all the oxygens (they all came from O2), you have X=C4H10

[dekoyl]

1. Why does have two singlets in ?

Thanks!

[TrueTears]

There are 2 environments of H

Look at the H attached to the O. (hydroxy group)

There are 2 hydroxy groups and they are both attached to the same 'type' of carbon. ie 1 environment

Now look at the H's attached on the Carbons ()

These H's are attached to a carbon which is attached to a OH for both groups. environment

Therefore 2 environments in total

[dekoyl]

Thank you TT.
I still suck at identifying carbon and hydrogen environments. I'll have to work on that.

[Dark Horse]

Yeah, I did this question too, however, I thought that the -CH2 would produce a triplet as there are two hydrogen atoms on the adjacent Carbon atom. However, the answers say that two singlets are formed. How is this the case?

[TrueTears]

The next to it is part of the SAME environment, splitting is caused by H's from different environments. Therefore it would stay as a singlet. Ratio would be 4:2 = 2:1 for the 2 environments.

[Mao]

For protons in the same environment, no spin-spin coupling occurs because the local magnetic field is the same. This can only occur for two adjacent groups on either side of the axis of symmetry.

[dekoyl]

^Thanks everyone.

Q: In mass spec., how do we know where the molecule fragments? In propan-1-ol, I wasn't sure if it's base peak was at 29 or 31.

Is this something we just have to know? Thanks =]

[TrueTears]

No you certainly do not know have remember the base peaks for each compound etc. Just have to know how to interpret a mass spectrum given. Eg base peak is the peak given a relative intensity of 100. So just look for that peak.

[dekoyl]

No you certainly do not know have remember the base peaks for each compound etc. Just have to know how to interpret a mass spectrum given. Eg base peak is the peak given a relative intensity of 100. So just look for that peak.

Ah cool, thanks TT.
It's just I have a question here:
"Given propan-1-ol undergoes complete fragmentation in a mass spectrometer, what's the m/e value of the base peak?"
I had a hard time choosing between 29 and 31.

(It's 31)

[shinny]

Consensus of chemistry teachers was that it was just a stupid question which couldn't really be solved at a VCE level.

[dekoyl]

Gah.. simple titration stuff.

Q2 VCAA 2005
Vitamin C and Iodine

A 20mL sample of the fruit juice was titrated up to 250 mL. 25 mL aliquots of the diluted fruit juice were titrated with a solution in which An average titre of 15.65 mL was obtained.

Q: Calculate the concentration of vitamin C in the original (undiluted) sample of fruit juice in mole per litre.

Why is it: and not as it was the aliquot that was 25 mL and not the original sample which was 20 mL?

Sorry if I'm unclear but it's a long question to type out

[TrueTears]

Is this sentence meant to "diluted up to"  -- "A 20mL sample of the fruit juice was titrated up to 250 mL"