2.2 - Relative mass

Question 1

What isotope is the standard isotope to base all atomic masses?

Answer 1

the carbon-12 isotope, which is the international standard for the measurement of atomic masses

Question 2

What is the mass of a carbon-12 isotope defined as exactly?

Answer 2

12 atomic mass units (12 u)

Question 3

What is the standard mass for atomic mass?

Answer 3

1u, which is the mass of 1/12th of an atom of carbon-12

Question 4

On this scale, what is the mass of 1u?

Answer 4

Approximately the mass of a proton or neutron?

Question 5

Define relative isotopic mass

Answer 5

Relative isotopic mass is the mass of an isotope relative to 1/12th the mass of an atom of carbon-12

Question 6

Does relative isotopic mass have any units and why or why not?

Answer 6

No units because it is a ratio of two masses.

Question 7

Define relative atomic mass (3)

Answer 7

The weighted mean mass of an atom of an element
relative to 1/12 th the mass
of a carbon-12 atom

Question 8

What does the weighted mean mass take into account?

Answer 8

It takes account of:

• the percentage abundance of each isotope
• the relative isotopic mass of each isotope

Question 9

How are percentage abundances of isotopes found experimentally?

Answer 9

The percentage abundances of the isotopes in a sample of an element are found experimental using a mass spectrometer

Question 10

What basic principle do mass spectrometers work upon?

Answer 10

1. Sample placed in mass spectrometer
2. Sample vaporised and then ionised to form positive ions
3. Ions then accelerated. Heavier ions move more slowly & are more difficult to deflect than lighter ions, so ions of each isotope separate
4. Ions detected on a mass spectrum as mass-to-charge ratio m/z (relative mass of ion / relative charge on ion)

Question 11

How are the ions accelerated?

Answer 11

Positive ions are attracted towards a negative plate, which is negatively charged and causes ions to accelerate

Question 12

How are the ions detected?

Answer 12

When ions arrive at the detector, flight times are recorded. At detector, positive ions pick up electron, which causes current to flow

• each ion reaching the detector adds to the signal, so the greater the abundance, the larger the signal

Question 13

How is the data from the signal that is detected analysed?

Answer 13

The detector passes a signal to the computer due to current, which generates a mass spectrum.