Monday, July 10, 2017

1.26: Calculate Relative Formula Masses (Including Relative Molecular Masses) (Mr) from Relative Atomic Masses (Ar)



RELATIVE ATOMIC MASS: Given by the symbol Ar
s
  • Calculated from the Mass Number and Relative Abundances of All the Isotopes of a particular Element

   

Example for Isotopes:
s
    The Table Shows Information about the Isotopes in a sample of Rubidium


ISOTOPE
NUMBER OF PROTONS
NUMBER OF NEUTRONS
PERCENTAGE OF ISOTOPE IN SAMPLE
1
37
48
72
2
37
50
28
 
    Use Information from the Table to Calculate the Relative Atomic Mass of   
    this Sample of Rubidium. Give your Answer to One Decimal Place

    (   72   x   85   )   +   (   28   x   87   )
    ____________________________     =   85.6
                             100

    Relative Atomic Mass = 85.6



s
RELATIVE FORMULA MASS: Given by the Symbol Mr
s
  • To calculate the Mr of a substance, you have to add up the Relative Atomic Masses of all the Atoms present in the formula
s
Example:
s
SUBSTANCE
ATOMS PRESENT
Mr
HYDROGEN
( H2 )
2 x H
( 2 x 1 ) = 2
WATER ( H2O )
( 2 x H ) + ( 1 x O )
( 2 x 1 ) + 16 = 18
POTASSIUM CARBONATE
( K2CO3 )
( 2 x K ) + ( 1 x C ) + ( 3 x O )
( 2 x 39 ) + 12 + ( 3 x 16 ) = 138
CALCIUM HYDROXIDE
(  Ca(OH)2  )
( 1 x Ca ) + ( 2 x O ) + ( 2 x H )
40 + ( 2 x 16 ) + ( 2 x 1 ) = 74
AMMONIUM SULFATE
( (NH4)2SO4  )
( 2 x N ) + ( 8 x H ) + ( 1 x S ) + ( 4 x O )
( 2 x 14 ) + ( 8 x 1 ) + 32 +
( 4 x 16 ) = 132

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