   Chapter 2.8, Problem 2.12CYU

Chapter
Section
Textbook Problem

The mass spectrum of phosphorus tribromide is illustrated below. Bromine has two stable isotopes, 79Br with abundances of 50.7% and 49.3%, respectively. (a) What molecular species give rise to the parent ion peaks at m/Z ratios of 270 and 272 (b) Explain why the relative abundances of the ions at m/Z ratios of 268 and 274 are approximately one-third of those at 270 and 272.

(a)

Interpretation Introduction

Interpretation:

For the mass spectrum of phosphorus tribromide, the molecular species give peaks at m/Z ratios of 270 and 272 has to be determined

Concept introduction:

Mass spectroscopy: It is a form of spectroscopic technique which is used for the elucidation of the molecules, depending upon the mass of the molecule.

Parent ion peak: It is defined as the prominent peak in the mass spectrum of the molecule which represents the not fragmented molecular ion in the given molecule with greater m/z value.

Base peak: It is the tallest peak in the spectrum.

PBr3 ionize to form positive ions. Some of them fragment into smaller ions.

Elements present in PBr3 are 79Br,81Brand31P

Explanation

m/Z ratios of 270 and 272 shows prominent peaks, so they are PBr3+

The elements present in PBr3 are 79Br,81Brand31P, so the m/Z (charge is taken as 1)

ratios of 31P and two different combination of 79Brand81Br are,

m/Zratio:31Pand2×<

(b)

Interpretation Introduction

Interpretation:

The relative abundance of ions at m/Z ratios of 268 and 274 are approximately one-third of those at 270 and 270 has to be explained.

Concept introduction:

Mass spectroscopy: It is a form of spectroscopic technique which is used for the elucidation of the molecules, depending upon the mass of the molecule.

Parent ion peak: It is defined as the prominent peak in the mass spectrum of the molecule which represents the not fragmented molecular ion in the given molecule with greater m/z value.

Base peak: It is the tallest peak in the spectrum.

PBr3 ionize to form positive ions. Some of them fragment into smaller ions.

Elements present in PBr3 are 79Br,81Brand31P.

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