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:
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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

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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

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Example:

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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