The Formula for a Hydrate Chemistry 11 5

The Formula for a Hydrate Chemistry 11. 5

Objectives n n Explain what a hydrate is and how its name reflects is composition. Determine the formula for a hydrate.

Key Terms n Hydrate

Naming Hydrates n n A compound that has a specific number of water molecules bound to its atoms. The number of water molecules is written following a dot. Na 2 CO 3 • 10 H 2 O n sodium carbonate decahydrate (Table 11 -1) Note: When calculating molar mass, the mass of water associated with the formula must be included.

Analyzing a Hydrate n n n To analyze a hydrate, the water associated with the compound must be driven off. Usually this is done by heating the material. The remaining substance is termed anhydrous: “without water. ”

Uses of Hydrates n n n Absorb water when in the anhydrous form. Added to fuel to keep water out the of mixture. Keeps electronics safe from humid air conditions.

Formula for a Hydrate n n Determine the number of moles of water associated with one mole of the hydrate. Example: Suppose you have a 5. 00 g sample of a hydrate of barium chloride. n Formula is Ba. Cl 2 • x. H 2 O. n After heating substance until water is driven off, mass is 4. 26 g.

Formula for a Hydrate n n 5. 00 g Ba. Cl 2 hydrate – 4. 26 g anhydrous Ba. Cl 2 = 0. 74 g H 2 O. Now convert the grams to mol. 4. 26 g Ba. Cl 2 x 1 mol Ba. Cl 2 = 0. 0205 mol Ba. Cl 2 208. 23 g Ba. Cl 2 0. 74 g H 2 O x 1 mol H 2 O = 0. 041 mol H 2 O 18. 02 g H 2 O

Formula for a Hydrate n n To determine the coefficient that precedes water in the formula for a hydrate: (moles water/moles compound) n 0. 041 mol H 2 O =2 0. 0205 mol Ba. Cl 2 n Ba. Cl 2 • 2 H 2 O

Practice Problems n Pg 340

Homework n Section 11. 5 Problems 33 and 34 on page 877