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: MgSO 4 7H 2 O

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

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

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 MgSO 4 7H 2 O: –magnesium sulphate heptahydrate MgSO 4 –anhydrous magnesium sulphate

MOLAR MASS OF HYDRATE The Molar mass of a hydrate INCLUDES the water molecules Find the M of CuCl 2 2H 2 O (35.45) + 2(18.02) = g/mol

MOLAR MASS OF HYDRATE Find the M of anhydrous CuCl (35.45) = g/mol THERE IS A DIFFERENCE!!! Hydrate weighs more!

% composition example… HYDRATE

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.

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

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 MgSO 4 xH 2 O - NEED: # H 2 O in MgSO 4 xH 2 O -Can calculate M of MgSO 4 = 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 g/mol = mol MgSO 4 RECALL: n = m M

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 = mol H 2 O RECALL: n = m M

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 = = Moles of H 2 O = = mol MgSO 4 : 7 mol H 2 O Therefore the formula is MgSO 4 7H 2 O

Homework Read Section 6.2 Hydrate Problems.