Many ionic compounds form crystals that trap a fixed number of water molecules inside their solid structure. These compounds are called hydrates, and the trapped water is called water of crystallization. Hydrates matter because the water changes the mass, color, and sometimes the properties of the salt.
Heating a hydrate can drive off the water and leave an anhydrous salt behind.
The formula of a hydrate shows the ratio of salt formula units to water molecules, such as CuSO4·5H2O. In the laboratory, the amount of water can be found by heating a known mass of hydrate until its mass stops changing. The mass lost is treated as water, and the remaining solid is the anhydrous salt.
Converting both masses to moles gives the mole ratio needed to write the hydrate formula.
Key Facts
- A hydrate is an ionic compound with water molecules built into its crystal structure.
- Water of crystallization is written after a centered dot, as in CuSO4·5H2O.
- Mass of water lost = mass of hydrate before heating - mass of anhydrous salt after heating.
- Moles = mass / molar mass.
- Hydrate formula ratio = moles of water : moles of anhydrous salt, converted to small whole numbers.
- Example heating reaction: CuSO4·5H2O(s) + heat -> CuSO4(s) + 5H2O(g).
Vocabulary
- Hydrate
- A crystalline ionic compound that contains a fixed number of water molecules in its structure.
- Water of crystallization
- Water molecules that are chemically included in a crystal lattice in a definite ratio.
- Anhydrous salt
- A salt that has no water of crystallization in its crystal structure.
- Formula unit
- The simplest whole-number ratio of ions in an ionic compound.
- Constant mass
- The point in a heating experiment when repeated heating and weighing gives the same mass, showing that water removal is complete.
Common Mistakes to Avoid
- Using mass ratio as the formula ratio is wrong because chemical formulas are based on mole ratios, not gram ratios.
- Forgetting to subtract the final mass from the initial mass is wrong because the water mass is the mass lost during heating, not the mass of the residue.
- Stopping after one heating is unreliable because some water may remain; heat, cool, and weigh repeatedly until the mass is constant.
- Including the dot water in the molar mass of the anhydrous salt is wrong when calculating moles of residue because the residue no longer contains that water.
Practice Questions
- 1 A 5.00 g sample of hydrated copper(II) sulfate is heated to constant mass and leaves 3.20 g of anhydrous CuSO4. Find the mass of water lost and the moles of water lost. Use H2O = 18.0 g/mol.
- 2 A hydrate of MgSO4 has mass 2.46 g. After heating, 1.20 g of anhydrous MgSO4 remains. Find the formula MgSO4·xH2O. Use MgSO4 = 120.4 g/mol and H2O = 18.0 g/mol.
- 3 A student heats a blue hydrate and obtains a white anhydrous powder. Explain why the color and mass changed, and describe one way to confirm that all water of crystallization was removed.