Relative Atomic Mass

Question 1

What is relative atomic mass (Ar)?

Answer 1

It is the average mass of an atom of an element relative to 1/12 the mass of an atom of carbon-12.

Question 2

What are the units for relative atomic mass?

Answer 2

g.mol^-1

Question 3

What is g.mol^-1?

Answer 3

e.g. oxygen is 16 g.mol^-1 so if you have one mole of oxygen atoms, it would weigh 16 grams

Question 4

What are not all isotopes?

Answer 4

they are not all integers - you can’t have 10.8 protons and neutrons

Question 5

What are isotopes?

Answer 5

atoms that have the same no. of protons but different no. of neutrons.

Question 6

To calculate the relative atomic mass, what two types of information do you need?

Answer 6

the mass numbers of the individual isotopes
their relative abundances (percentages)

(mass spectroscopy can give this info)

Question 7

What is it possible to have for one element?

Answer 7

different isotopes

e.g. chlorine can have more than 1 isotope

Question 8

How do you calculate Ar?

Question 9

How do you calculate Ar?

Answer 9

(Mass Isotope 1 x % Abundance) + (Mass Isotope 2 x %Abundance) / 100

Question 10

An example of calculating Ar

Answer 10

e.g., Cl = (35x75) + (37x25) / 100 = 35.5 g.mol^-1

Question 11

If abundance values are not %, but just relative values, what do you do?

Answer 11

divide by the sum of those values