Saturday, 30 March 2013

1.16 calculate relative formula masses (Mr) from relative atomic masses (Ar)

Mr is relative formula mass, it is the mass of a molecule. Its calculation is (number of that element in the molecule x relative atomic mass) + (number of that element in the molecule x relative atomic mass)

Ar is relative atomic mass, it is the average mass of an atom of a specific element. It is calculated by doing (% of isotope x mass of isotope) + (% of isotope x mass of isotope) over 100.

7 comments:

  1. Hannah, Could you give me an example please?

    ReplyDelete
    Replies
    1. Cl is 25% 35 and 75% 37 so
      (25 x 35) + (75 x 37) over 100=
      (875) + (2775) over 100=
      3650 / 100= 36.5

      So the Ar of Cl is 36.5
      and the Ar of H is 1
      to work out the Mr of HCl

      (1 x 1) + (1 x 36.5)=
      1 + 36.5=
      37.5

      Delete
    2. The percentage of isotopes in a sample of Cl atoms is actually the other way around:
      25% Cl37 atoms 75% Cl35 atoms

      So the Ar of Chlorine is actually 35.5

      Thanks for everything you've done Hanna, amazing resource

      Delete
    3. This comment has been removed by the author.

      Delete
  2. don't you mean it is the mass of a compound as a molecule can just be one element but a compound is at least two different elements

    ReplyDelete