Ch. 2.2 : Relative Mass

Question 1

How and why is the unit ‘u’ used?

Answer 1

Kg would be awkward to work with.
The ass of a carbon - 23 isotope is defined as exactly 12 atomic mass units ( u ).
The standard mass for atomic mass is 1 u, the mass of 1/12th of an atom of carbon - 12.

Question 2

Define relative isotopic mass.

Answer 2

The mass of an isotope relative to 1/12th of the mass of an atom of carbon - 12.

Question 3

Define relative atomic mass.

Answer 3

Ar is the weighted mean mass of an atom of an element relative to 1/12th of the mass of an atom of carbon 12.

Question 4

What are the two things the weighted mean mass takes account of?

Answer 4

• the percentage abundance of each isotope

- the relative isotopic mass of each isotope

Question 5

What does a mass spectrometer do?

Answer 5

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

Question 6

How do mass spectrometers work briefly ?

Answer 6

• a sample is placed in the mass spectrometer
• the sample is vaporised and then ionised to form positive ions
• the ions are accelerated. Heavier ions move slower and are more difficult to detect than lighter ions, so the ions of each isotope are separated.
• the ions are detected as a mass - to - charge ratio m/z. Each ion reaching the detector adds to the signal, so the greater the abundance, the larger the signal.

Question 7

How do you find the mass to charge ratio?

Answer 7

Relative mass of ion / relative charge of ion

Question 8

How do you work out relative atomic mass?

Answer 8

Abundance x mass
_____________________
100

Do this for every isotope and add them all together

Question 9

How do you work out % abundance ?

Answer 9

Peak height
_____________ x 100
Total height