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
HYDRATES
FORMULA
CaSO4 • 2H2O
CaCl2 • 2H2O
LiCl2 • 4H2O
MgSO4 • 7H2O
Ba(OH)2 • 8H2O
Na2CO3 • 10H2O
CHEMICAL NAME
HYDRATES
FORMULA
CHEMICAL NAME
CaSO4 • 2H2O
Calcium sulphate dihydrate
CaCl2 • 2H2O
Calcium chloride dihydrate
LiCl2 • 4H2O
Lithium chloride
tetrahydrate
Magnesium sulphate
heptahydrate
Barium hydroxide
octahydrate
Sodium carbonate
decahydrate
MgSO4 • 7H2O
Ba(OH)2 • 8H2O
Na2CO3 • 10H2O
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
RECALL:
n = m
M
MgSO4
RECALL:
moles of H2O =
mass H2O
n = m
Molar mass H2O
M
WE know Mass of Water from part (a) 
25.6 g
= 25.6 g
2(1.01) + 16 g/mol
= 1.42064 mol H2O
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
0.20269
= 1
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.