MOLECULAR AND EMPIRICAL FORMULA OF A HYDRATE Section

MOLECULAR AND EMPIRICAL FORMULA OF A HYDRATE Section: 6. 2

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

HYDRATES FORMULA Ca. SO 4 • 2 H 2 O Ca. Cl 2 • 2 H 2 O Li. Cl 2 • 4 H 2 O Mg. SO 4 • 7 H 2 O Ba(OH)2 • 8 H 2 O Na 2 CO 3 • 10 H 2 O CHEMICAL NAME

HYDRATES FORMULA CHEMICAL NAME Ca. SO 4 • 2 H 2 O Calcium sulphate dihydrate Ca. Cl 2 • 2 H 2 O Calcium chloride dihydrate Li. Cl 2 • 4 H 2 O Lithium chloride tetrahydrate Mg. SO 4 • 7 H 2 O Magnesium sulphate heptahydrate Ba(OH)2 • 8 H 2 O Barium hydroxide octahydrate Na 2 CO 3 • 10 H 2 O Sodium carbonate decahydrate

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

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

MOLAR MASS OF HYDRATE • The Molar mass of a hydrate INCLUDES the water molecules • Find the M of Cu. Cl 2 • 2 H 2 O 63. 55 + 2(35. 45) + 2(18. 02) = 170. 49 g/mol

MOLAR MASS OF HYDRATE • Find the M of anhydrous Cu. Cl 2 63. 55 + 2(35. 45) = 134. 45 g/mol THERE IS A DIFFERENCE!!! Hydrate weighs more!

% composition example… HYDRATE

A 50. 0 g sample of hydrate contains 24. 4 g Mg. SO 4. a) Calculate the percent by mass of water in Mg. SO 4 • x H 2 O.

a) Mass percent of water in Mg. SO 4 • x H 2 O Given: formula Mg. SO 4 • x H 2 O total mass of sample: 50. 0 g mass of Mg. SO 4: 24. 4 g Therefore, the mass of water = total mass of sample – mass Mg. SO 4 = 50. 0 g – 24. 4 g = 25. 6 g H 2 O

Mass percent of water = = = mass of water x 100% total mass of sample 25. 6 g x 100% 50. 0 g 51. 2 %

b) Find the value of x in Mg. SO 4 • x. H 2 O - NEED: # H 2 O in Mg. SO 4 • x. H 2 O -Can calculate M of Mg. SO 4 = 120. 38 g/mol - Can calculate moles of Mg. SO 4 and H 2 O moles of Mg. SO 4 = = = mass Mg. SO 4 Molar mass Mg. SO 4 24. 4 g 120. 38 g/mol 0. 20269 mol RECALL: n = m M Mg. SO 4

RECALL: moles of H 2 O = mass H 2 O n = m Molar mass H 2 O M 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

To find the formula of Mg. SO 4 • x. H 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 Mg. SO 4 = 0. 20269 = 1 Moles of H 2 O = 1. 42064 = 7 0. 20269 1 mol Mg. SO 4 : 7 mol H 2 O Therefore the formula is Mg. SO 4 • 7 H 2 O

Homework Read Section 6. 2 Hydrate Problems.