How to Correct for Water in a Formula Weight of a Chemical

Many chemicals, especially inorganic compounds and minerals, contain water as an integral part of their crystalline phase. Some examples of hydrates are cupric sulfate pentahydrate, CuSO4•5H2O and magnesium chloride hexahydrate, MgCl2 •6H2O. Correction for the hydrate water is necessary to determine accurately the concentration of chemicals. As an example, calculate molar mass (formula weight) correction for magnesium chloride hexahydrate.

Things You'll Need

• Periodic table of the chemical elements
• Calculator

Instructions

1. Calculate the molar mass of the chemical. Molar mass is calculated as the sum of mass of all atoms in the molecule. Atomic weights of corresponding elements are given in the periodic table of the chemical elements (see link in Resources).
   In our example with magnesium chloride:
   Molar mass (MgCl)=M(Mg)+2 x M(Cl)=24+2 x 35.5=95 g/mole.

2. Multiply 18 g/mole by the number of water molecules to calculate the molar mass of the hydrate water. In our example, magnesium chloride hexahydrate contains 6 water molecules and molar mass (water) equal to 18 x 6 = 108 g/mole.

3. Add up the molar masses of the chemical (Step 1) and the hydrate water (Step 2) to calculate molar mass of the chemical corrected for hydrated water. In our example, it is 108 + 95 = 203 g/mole. Note that the molar mass of magnesium chloride hydrate increases more than twice due to the water.

Tips & Warnings

• The term "formula weight" is now obsolete; "molar mass" is used instead.

References

• Chemistry; K.W. Whitten, R.E. Davis, L. Peck, and G.G. Stanley, Brooks Cole, 2009.

Resources

• Periodic table of the Chemical Elements
• Encyclopedia: Hydrate