Hydrates Unit 6 Lesson 3

Hydrates Ionic compound that has water incorporated into its crystal structure

Hydrates Written as MgSO4●7H2O Read as “Magnesium Sulfate Heptahydrate” Means for every one MgSO4 you have 7 water molecules. Notice the prefixes we learned for naming covalent compounds are used to tell how many H2O molecules you have If finding the molar mass, include the water!

Hydrate Practice Name the following Hydrates and find their molar masses CuSO4 • 5H2O ZnCl2 • 3H2O Mg(NO3)2• 4H2O

Determining the Formula of a Hydrate Example: Find the formula of a hydrate that is 48.8% MgSO4 and 51.2% H2O Step 1 – assume exactly 100 grams 48.8 g MgSO4 and 51.2 g H2O Step 2 – calculate moles 48.8 g MgSO4 (1 mol/120.38g) = .405 Moles MgSO4 51.2 g H2O (1mol/18.02g) = 2.84 moles H2O Step 3 – calculate mole ratio by dividing by smallest number of moles .405/.405 = 1 mole MgSO4 2.84/.405 = 7.01 moles H2O So there are 7 moles of water per 1 mole of MgSO4 MgSO4●7H2O

Example A hydrated Cobalt (II) Chloride sample has a mass of 11.75g. After being heated (to remove all water) the mass is 9.25 g. What is the formula for the hydrate? 9.25 g CoCl2 and 2.50 g H2O 0.0712 moles CoCl2 and 0.139 moles H2O 0.0712/0.0712 = 1 mole CoCl2 0.139/0.0712 = 1.95 moles H2O CoCl2 •2H2O

Example: Finding mass percent from a hydrate formula Determine the mass percent of the anhydrous portion and the water in the hydrate Na3PO4 •10H2O Find the mass of the anhydrous portion: 3(22.99g) + 30.97g + 4(16.00g) = 163.94g Find the mass of the ten attached waters: 10(18.016g) = 180.16g Find the mass of the entire hydrate: 163.94g + 180.016g = 344.06g Mass percent of the anhydrous: (163.94g/344.06g)100 = 47.649% anhydrous Mass percent of the water: (180.16g/344.06g)100 = 52.351% water