Mass spectrometry and relative masses of atoms, isotopes and molecules

Question 1

the relative atomic (Ar) mass of an atom is

Answer 1

the weighed mean mass of an element compared to 1/12 of the mass of an atom of carbon-12

Question 2

the hydrogen atom is the

Answer 2

lightest atom, so is given the value 1 and used as the standard comparative

Question 3

The discovery of the isotope meant ……could no longer be used as the standard, so ……… is used instead

Answer 3

hydrogen, carbon-12

Question 4

relative isotopic mass is

Answer 4

the mass of an individual atom of a particular isotope relative to 1/12 of the mass of an atom of carbon-12

Question 5

how do we calculate relative atomic mass using abundances?

Answer 5

(relative isotope mass of substance A x % abundance of A) + (relative isotopic mass of substance B x % abundance of B) all divided by the sum of % abundances

Question 6

how does a mass spectrometer work?

Answer 6

it produces positive ions that are defected by a magnetic field according to their mass-to-charge ratio (m/z). It also calculates the relative abundance of each positive ion and displays this as a percentage.

Question 7

the positive ions used in mass spectrometry could be

Answer 7

positively charged atoms, positively charged molecules or positively charged fragments of molecules

Question 8

to calculate the relative atomic mass of an element from a mass spectrometry graph we need to

Answer 8

look at the relative peak heights on the graph, creating a ratio for peaks in correspondence to the relative isotopic mass obtained in mass spectrometry to get the abundance of each isotope.

Question 9

the two ways of calculating relative molecular mass are;

Answer 9

by using the abundances of each isotope in the molecule and the probability of each isotope bonding with another to form a molecule to work out the relative molecular mass or
by adding the relative atomic masses of each atom in the molecule

Question 10

the molecular ion peak is

Answer 10

the peak with the highest m/z ratio in the mass spectrum, the M peak.

Question 11

however, the molecular ion peak, M peak, is not always the last peak on the spectrum because

Answer 11

in some organic compounds, particularly ones with large masses, there is a proportion of carbon-13 isotopes present, which show as a shorter peak on the right of the M peak, which is called the M + 1 peak

Question 12

the M + 1 peak is

Answer 12

a product of the presence of carbon-13 isotopes in organic compounds