There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine consists of molecules, and the mass of a molecule is the sum of the masses of the two atoms in the molecule. The mass spectrum of consists of three peaks: m/zRelative Peak Intensity157.836 0.2569 159.834 0.4999 161.832 0.2431
c. Determine the average molecular mass of a molecule.

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Determine the isotopes involved: The mass spectrum peaks suggest the presence of two isotopes of bromine, typically Br-79 and Br-81.

Identify the molecular combinations: The peaks correspond to Br-79/Br-79, Br-79/Br-81, and Br-81/Br-81.

Assign the m/z values to these combinations: 157.836 for Br-79/Br-79, 159.834 for Br-79/Br-81, and 161.832 for Br-81/Br-81.

Calculate the average molecular mass: Use the relative peak intensities as weights to find the weighted average of the molecular masses.

Apply the formula: Average molecular mass = (157.836 * 0.2569) + (159.834 * 0.4999) + (161.832 * 0.2431).

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Isotopes

Isotopes are variants of a chemical element that have the same number of protons but different numbers of neutrons, resulting in different atomic masses. For bromine, the two stable isotopes are bromine-79 and bromine-81. Understanding isotopes is crucial for calculating average atomic masses, as the relative abundance of each isotope affects the overall mass of the element.

Mass Spectrum

A mass spectrum is a graphical representation of the mass-to-charge ratio (m/z) of ions, which helps identify the composition of a sample. Each peak in the spectrum corresponds to a specific ion, with its height indicating the relative abundance of that ion. Analyzing the mass spectrum of bromine allows us to determine the contributions of its isotopes to the average molecular mass.

Average Molecular Mass

The average molecular mass of a compound is calculated by taking the weighted average of the masses of its isotopes, based on their relative abundances. This involves multiplying the mass of each isotope by its relative peak intensity from the mass spectrum and summing these products. This concept is essential for understanding the overall mass of molecules in elemental bromine.

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Textbook Question

Gallium (Ga) consists of two naturally occurring isotopes with masses of 68.926 and 70.925 amu. a. How many protons and neutrons are in the nucleus of each isotope? Write the complete atomic symbol for each, showing the atomic number and mass number. b. The average atomic mass of Ga is 69.72 amu. Calculate the abundance of each isotope.

Textbook Question

There are two different isotopes of bromine atoms. Under normal conditions, elemental bromine consists of molecules, and the mass of a molecule is the sum of the masses of the two atoms in the molecule. The mass spectrum of consists of three peaks: m/zRelative Peak Intensity157.836 0.2569 159.834 0.4999 161.832 0.2431

a. What is the origin of each peak (of what isotopes does each consist)?

Textbook Question

b. What is the mass of each isotope?

Textbook Question

d. Determine the average atomic mass of a bromine atom.

Textbook Question

e. Calculate the abundances of the two isotopes.

Textbook Question

It is common in mass spectrometry to assume that the mass of a cation is the same as that of its parent atom. (b) What percentage of the mass of an 1H atom does the electron represent?