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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Charismaztex on January 29, 2010, 09:18:46 PM

Title: Questions on mass spectrums
Post by: Charismaztex on January 29, 2010, 09:18:46 PM

A) Why is the height of molecular bromine in the mass spectrum at 158, 160, and 162 in the ratio 1:2:1? [2 marks] (I know that Br-79 and Br-81 exist in the ratio of 1:1 but I can't seem to make the connection with Br2)

B) Why are two molecular ion peaks found in the mass spectrum of chloroethane, at m/z values of 64 and 66, with relative intensities in the ratio of approximately 3 to 1, respectively? [4 marks] ( I'm thinking it's to do with the isotopes of Cl-35 and Cl-37, existing in the ratio of 3:1, but if someone can confirm this, it would be great.)

C) Why, in the case of Ch2Br2, significant peaks appear in the mass spectrum at m/z values of 172, 176, and 174? [4 marks] (I'm thinking it's to do with the possibility of combination of Br-79 with Br-79, Br-81 with Br-79, and Br-81 with Br-81, but if someone can give a full answer worth 4 marks it would be great also.)

Thanks in advance,
Charismaztex

Title: Re: Questions on mass spectrums
Post by: Wreath on January 30, 2010, 03:26:52 AM

A) you correctly blame the isotopes. you can has 3 types of Br₂: 79 - 79, 79 - 81, 81 - 81. count the probabilities :-)
B) i think i would confirm  ;)
C) in my opinion, you should assume the ^{12, 13, 14}C in combination with both bromines.

Title: Re: Questions on mass spectrums
Post by: MOTOBALL on January 30, 2010, 03:59:08 PM

(A)  Br-a 79 and Br-b 79 => m/z 158
      Br-a 79 and Br-b 81 => m/z 160
      Br-a 81 and Br-b 79 => m/z 160
      Br-a 81 and Br-b 81 => m/z 162

Thus ratio is 1:2:1

(B) correct

(C) CH2 = 14 Da; Br2 = 158, 160, 162 from (A) above
Thus CH2Br2 gives m/z 172, 174, 176 in ratio 1:2:1

MOTOBALL

Title: Re: Questions on mass spectrums
Post by: Charismaztex on January 31, 2010, 12:57:58 AM

Thank you all for your help  :)