Ch. 10: The Mole Sec. 10.5: The Formula for a Hydrate
Formula for a Hydrate • Determine the formula for a hydrate from laboratory data.
Formula for a Hydrate • A hydrate is a compound that has a specific number of water molecules bound to its atoms.
Formula for a Hydrate • In the formula for a hydrate, the number of water molecules associated with each formula unit of the ionic compound is written following a dot Na2CO3•10H2O • This is the same thing as saying that for every 1 mole of Na2CO3, there are 10 moles of water. There is a 1:10 mole ratio.
Formula for a Hydrate • We know that to analyze the amount of water in a hydrate you must drive off the water of hydration. • Often this is done by heating the compound. • The substance remaining is anhydrous or “without water”.
Formula for a Hydrate • Suppose you have a 5.00 g sample of a hydrate of cobalt chloride. • Its formula is CoCl2•xH2O • The value of x indicates the number of moles of water associated with 1 mole of cobalt chloride. X must be determined.
Formula for a Hydrate • The sample of the hydrate is heated to drive off the water of hydration. • The mass of the anhydrous cobalt chloride is determined. It is 2.72 g.
Formula for a Hydrate • The mass of the water of hydration is the difference between the mass of the hydrate (5.00 g) and the mass of the anhydrous compound (2.80 g). • 5.00 g – 2.72 g = 2.28 g H2O
Formula for a Hydrate • This means there was 2.72 g of CoCl2 and 2.28 g of H2O in the hydrate. • Those masses must now be converted to moles. • 2.72 g CoCl2 x 1 mole = 0.0209 mol CoCl2 129.9 g • 2.28 g H2O x 1 mole = 0.127 mol H2O 18 g
Formula for a Hydrate • Now the following formula must be used to find x in CoCl2•xH2O x = molesH2O/moles anhydrous compound so, X = .127/.0209 = 6 • This means 6 moles of water are associated with 1 mole of cobalt chloride. • The formula is CoCl2 • 6H2O • The name: cobalt(II) chloride hexahydrate.
Practice Problems • 2.5 g of a copper sulfate hydrate (CuSO4 • xH2O) is heated. After heating, the sample is massed at 1.59 g. What is the formula and name of this hydrate? • A hydrate is found to have the following percent composition: 48.8% MgSO4 and 51.2% H2O. What is the formula and name for this hydrate?