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.