Na2B4O7 10H2O
Goal:
Given experimental masses, be able to calculate the formula of a hydrate.
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Determining the Molecular Formula of a Hydrate
Hydrate: A compound that has water molecule(s) incorporated into its crystal structure. The compound is anhydrous when it does not have water in its crystal structure. When determining a hydrated compound's molar mass, the mass of the water must be included.
Eg. Molar mass of anhydrous magnesium sulfate (MgSO4) = 120.38 g/mol
Molar mass of magnesium sulfate heptahydrate (MgSO4 7H2O) = 246.52 g/mol
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Sample Problem:
A hydrate of barium hydroxide, Ba(OH)2xH2O, has a mass 50.0 g. If the mass of the anhydrous barium hydroxide is 27.2 g.
a) Calculate the mass % of water
b) Determine the value of x in the formula.
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b) Use a table to find the x value
For every 1 Ba(OH)2 formula unit, there are 8 water molecules. x=8
The formula of the hydrate is Ba(OH)2 ∙ 8H2O
Homework
p. 225 # 23, 24, 25 p. 228 # 6 4