Structure 1.4 – Counting particles by mass : The mole

S1.4.2 – Relative atomic mass and relative formula mass

📌 Relative atomic mass

  • The relative atomic mass is the weighted average of different isotopes
  • The standardised relative point for comparison is the C12 atom
  • Relative atomic mass is given by the symbol Ar and has no units
  • The relative formula mass for an element can be found in the periodic table

📌 Relative formula mass

  • Relative formula mass is the sum of all the relative atomic masses in a compound (give by the symbol Mr)

Example

QUESTION : Calculate the formula mass for CuSO4

SOLUTION : Ar Cu = 63.5

Ar S = 32.1

Ar 0 = 16.00

Since there are 4 oxygen atoms : 16 x 4 = 64

Mr = 63.5 + 32.1 + 64 = 159.6