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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
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.
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