Spectroscopy help

[mandy]

The presence of methanol can be confirmed by its mass spectrum, shown below.

[IMG]http://img689.imageshack.us/img689/9311/53071017.png[/img]

Label each peak of the spectrum with the formula of the ion.

How would I do this question? Am I right to say that the peak at 15 is caused by the CH₃?

P.S. Sorry about the ugly graph, lol. No mouse control ==

[happyhappyland]

I havnt covered mass spec yet but Ive breifly read about it. I think you just label each peak by saying that this peak is caused by this structure, like you said at 15 it is Ch3 so you write ch3 on top of that peak etc.

[chem-nerd]

32 - CH₃OH

31 - CH₂OH⁺

29 - CHO⁺

15 - CH₃⁺

[mandy]

32 - CH₃OH

31 - CH₂OH⁺

29 - CHO⁺

15 - CH₃⁺

For these kinds of questions, can I just split it up anywhere, as long as I don't mess up the bonding and which ion bonds to which?
Thank you for helping me

[mandy]

Also, this isn't Spectroscopy, but how would I do this question:

0.132g of a pure carboxylic acid (R-COOH) was dissolved in 25.00mL of water and titrated with 0.120M NaOH solution. A volume of 14.80mL was required to reach the endpoint of the titration.
The carboxylic acid could be:
a. HCOOH
b. CH₃COOH
c. C₂H₅COOH
d. C₃H₇COOH

I found the mole of the NaOH, but then now I have no idea.

[simonhu81292]

is the answer C?

[fady_22]

Once you find the mole of NaOH, you can find the mole of the carboxylic acid (as it is a monoprotic acid, the ratio of NaOH:acid will be 1:1, and so the n(acid)=n(NaoH).
Using the number of mole of the carboxylic acid, you can find its molar mass (where , as the mass is given). You can now compare your answer to the molar masses of each possible answer, and choose the right compound.

[mandy]

is the answer C?

It's a worksheet my tutor gave me and he doesn't give out solutions, so I don't know what the answer is :S

[fady_22]

I got C too.

[mandy]

Once you find the mole of NaOH, you can find the mole of the carboxylic acid (as it is a monoprotic acid, the ratio of NaOH:acid will be 1:1, and so the n(acid)=n(NaoH).
Using the number of mole of the carboxylic acid, you can find its molar mass (where , as the mass is given). You can now compare your answer to the molar masses of each possible answer, and choose the right compound.

Okay, I did what you said, and I got a molar mass of 74.32, which is basically the molar mass of C₂H₅COOH, c.
Thank you guyssss !

[Mao]

32 - CH₃OH

For the sake of being pedantic:
32 - CH₃OH⁺

[Mao]

32 - CH₃OH

31 - CH₂OH⁺

29 - CHO⁺

15 - CH₃⁺

For these kinds of questions, can I just split it up anywhere, as long as I don't mess up the bonding and which ion bonds to which?
Thank you for helping me

A few things to remember:


- Only the +ve charged ions will bend and reach the detector

- For a molecule AB, it can be broken into A⁺ + B or A + B⁺ [usually only one bond is broken]. Generally, electronegative elements (oxygen mainly) tend to stay attached to the carbon, and usually keep the electron if the bond ever gets broken (does not form a cation). This is why you don't see a peak at 17 (OH+). Weaker bonds (C-C) tend to break before stronger bonds (C-H)

- For R₁R₂, you can get almost equal share of R1+ and R2+ (both are hydrocarbon chains)
- For R-CH2OH, R+ and CH2OH+ will be abundant
- For R1-CH(OH)-R2, you can get R1+, R2+, or lose a H2O.

Read this if you feel like a slightly more detailed explanation http://www.cem.msu.edu/~reusch/VirtualText/Spectrpy/MassSpec/masspec1.htm#ms4



BTW, most probably beyond the syllabus

[mandy]

Wow, thank you for the help Mao, that link is really in depth and helpful