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Do you include the coefficient for molar mass?

  • Students get a little confused sometimes whether or not to include the coefficient when determining Molar Mass.
  • Molar mass means the mass of 1 mole of substance
  • You just have to multiply the atomic mass of each individual atom by the subscripts and add them up
  • The molar mass of water is always (2 x H) + (1 x O) = 18g/mol
  • However, if you want to know what the mass of 6 moles of water is, then the total mass is (6mol x 18g/mol) = 108g.

When calculating molar mass, do you include the coefficient?


Do you include the coefficient of water molecules in hydrates?

  • Should you include the coefficient of water molecules
    when you calculate the molar mass of a hydrate such as Magnesium sulfate heptahydrate?
  • The answer is YES!
  • Even though there are 7 molecules of water,
    the water is incorporated into ONE MOLE of the
    solid ionic compound.
  • Mg=24.305
    S=32.06
    O=16.0
    H=1.0
    Multiply each atomic weight by the total no of each atom:
    MgSO4.7H2O –> 1 Mg, 1 S, 11 O, 14 H
  • Molar mass = (1 x 24.305) + (1 x 32.06) + (11 x 16) + (14 x 1)
  • The molar mass of magnesium sulfate heptahydrate is 246.365 g/mol

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