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Published byIlene Allison Modified over 9 years ago
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MOLECULAR AND EMPIRICAL FORMULA OF A HYDRATE Section: 6.2
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HYDRATES Recall: Formula units (molecules) that have a specific number of water molecules attached Ex: MgSO 4 7H 2 O
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HYDRATES FORMULACHEMICAL NAME CaSO 4 2H 2 O CaCl 2 2H 2 O LiCl 2 4H 2 O MgSO 4 7H 2 O Ba(OH) 2 8H 2 O Na 2 CO 3 10H 2 O
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HYDRATES FORMULACHEMICAL NAME CaSO 4 2H 2 O Calcium sulphate dihydrate CaCl 2 2H 2 O Calcium chloride dihydrate LiCl 2 4H 2 O Lithium chloride tetrahydrate MgSO 4 7H 2 O Magnesium sulphate heptahydrate Ba(OH) 2 8H 2 O Barium hydroxide octahydrate Na 2 CO 3 10H 2 O Sodium carbonate decahydrate
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HYDRATES A chemist may know the formula of the ionic part of the hydrate but not how many water molecules are present
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RECALL… When the water molecule is removed the compound becomes ANHYDROUS MgSO 4 7H 2 O: –magnesium sulphate heptahydrate MgSO 4 –anhydrous magnesium sulphate
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MOLAR MASS OF HYDRATE The Molar mass of a hydrate INCLUDES the water molecules Find the M of CuCl 2 2H 2 O 63.55 + 2(35.45) + 2(18.02) = 170.49g/mol
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MOLAR MASS OF HYDRATE Find the M of anhydrous CuCl 2 63.55 + 2(35.45) = 134.45 g/mol THERE IS A DIFFERENCE!!! Hydrate weighs more!
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% composition example… HYDRATE
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A 50.0 g sample of hydrate contains 24.4 g MgSO 4. a) Calculate the percent by mass of water in MgSO 4 x H 2 O.
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a) Mass percent of water in MgSO 4 x H 2 O Given: formula MgSO 4 x H 2 O total mass of sample: 50.0 g mass of MgSO 4 : 24.4 g Therefore, the mass of water = total mass of sample – mass MgSO 4 = 50.0g – 24.4 g = 25.6 g H 2 O
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Mass percent of water = mass of water x 100% total mass of sample = 25.6 g x 100% 50.0 g = 51.2 %
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b) Find the value of x in MgSO 4 xH 2 O - NEED: # H 2 O in MgSO 4 xH 2 O -Can calculate M of MgSO 4 = 120.38 g/mol - Can calculate moles of MgSO 4 and H 2 O moles of MgSO 4 = mass MgSO 4 Molar mass MgSO 4 = 24.4 g 120.38 g/mol = 0.20269 mol MgSO 4 RECALL: n = m M
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moles of H 2 O = mass H 2 O Molar mass H 2 O WE know Mass of Water from part (a) 25.6 g = 25.6 g 2(1.01) + 16 g/mol = 1.42064 mol H 2 O RECALL: n = m M
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To find the formula of MgSO 4xH 2 O Use RATIO: * RECALL: To find ratio Identify the lowest mole value and divide both mole values with it to find the ratio* Mole of MgSO 4 = 0.20269 = 1 0.20269 Moles of H 2 O = 1.42064 = 7 0.20269 1 mol MgSO 4 : 7 mol H 2 O Therefore the formula is MgSO 4 7H 2 O
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Homework Read Section 6.2 Hydrate Problems.
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