MOLECULAR AND EMPIRICAL FORMULA OF A HYDRATE Section: 6.2.

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: MgSO4 • 7H2O

HYDRATESFORMULA CHEMICAL NAME

CaSO4 • 2H2O

CaCl2 • 2H2O

LiCl2 • 4H2O

MgSO4 • 7H2O

Ba(OH)2 • 8H2O

Na2CO3 • 10H2O

HYDRATESFORMULA CHEMICAL NAME

CaSO4 • 2H2O Calcium sulphate dihydrate

CaCl2 • 2H2O Calcium chloride dihydrate

LiCl2 • 4H2O Lithium chloride tetrahydrate

MgSO4 • 7H2O Magnesium sulphate heptahydrate

Ba(OH)2 • 8H2O Barium hydroxide octahydrate

Na2CO3 • 10H2O 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

• MgSO4 • 7H2O: – magnesium sulphate heptahydrate

• MgSO4

– anhydrous magnesium sulphate

MOLAR MASS OF HYDRATE

• The Molar mass of a hydrate INCLUDES the water molecules

• Find the M of CuCl2 • 2H2O

63.55 + 2(35.45) + 2(18.02) = 170.49g/mol

MOLAR MASS OF HYDRATE

• Find the M of anhydrous CuCl2

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 MgSO4. a) Calculate the percent by mass of water in MgSO 4 • x H2O.

a) Mass percent of water in MgSO4• x H2O

Given: formula MgSO4• x H2O total mass of sample: 50.0 g mass of MgSO4: 24.4 g

Therefore, the mass of water = total mass of sample – mass MgSO4

= 50.0g – 24.4 g = 25.6 g H2O

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 MgSO4 • xH2O

- NEED: # H2O in MgSO4•xH2O- Can calculate M of MgSO4 = 120.38 g/mol - Can calculate moles of MgSO4 and H2O

moles of MgSO4 = mass MgSO4

Molar mass MgSO4

= 24.4 g 120.38 g/mol

= 0.20269 mol MgSO4

RECALL:n = m M

moles of H2O = mass H2O

Molar mass H2O

WE know Mass of Water from part (a) 25.6 g

= 25.6 g 2(1.01) + 16 g/mol

= 1.42064 mol H2O

RECALL:n = m M

To find the formula of MgSO4 •xH2O

Use RATIO: * RECALL: To find ratio Identify the lowest mole value and divide both mole values with it to find the ratio*

Mole of MgSO4 = 0.20269 = 1 0.20269

Moles of H2O = 1.42064 = 7 0.20269

1 mol MgSO4 : 7 mol H2O

Therefore the formula is MgSO 4

•7H2O

Homework

Read Section 6.2 Hydrate Problems.