Hydrates

• Water molecules can adhere to ions as the solid forms

• The water becomes trapped and is part of the crystal

• Opals and other gems are common hydrates & get their color from the water.

• Hydrates have a specific amount of water associated with each formula unit

Hydrates

• The amount of water is indicated in the formula: Na2CO3

.10H2O

• This means there are 10 moles of water for every mole of the ionic compound or 10 molecules of water for each formula unit

• The water must be calculated into the molar mass of the substance

• The name is sodium carbonate decahydrate

Formula for a Hydrate• Need to determine the number of moles of

water associated with one mole of the hydrate

• Heat the hydrate to drive off all water and form the anhydrous compound

• Use the difference in mass between the hydrate and anhydrous compounds to determine the mass of water & the cpd

• Use these masses to calculate the moles of each then find the ratio between them

Example

• You have a 5.00 g sample of a hydrate of barium chloride. After heating, the dried barium chloride has a mass of 4.26 g. What is the formula and name for this hydrate?

Try It1. A mass of 2.50 g of blue, hydrated

copper (II) sulfate is placed in a crucible and heated. After heating, 1.59 g white anhydrous copper (II) sulfate remains. What is the formula and name for the hydrate?

2. Gypsum is hydrated calcium sulfate. A 4.89 g sample of this hydrate was heated, and after the water was driven off, 3.87 g anhydrous calcium sulfate remained. Determine the formula and name the compound.