1. MOLECULAR AND EMPIRICAL FORMULA OF A HYDRATE Section: 6.2

2. HYDRATES Recall: Formula units (molecules) that have a specific number of water molecules attached Ex: MgSO 4 7H 2 O

3. 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

4. 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

5. HYDRATES A chemist may know the formula of the ionic part of the hydrate but not how many water molecules are present

6. RECALL… When the water molecule is removed the compound becomes ANHYDROUS MgSO 4 7H 2 O: –magnesium sulphate heptahydrate MgSO 4 –anhydrous magnesium sulphate

7. 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

8. 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!

9. % composition example… HYDRATE

10. 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.

11. 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

12. Mass percent of water = mass of water x 100% total mass of sample = 25.6 g x 100% 50.0 g = 51.2 %

13. 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

14. 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

15. 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

16. Homework Read Section 6.2 Hydrate Problems.