Molecular and Empirical Formula of a Hydrate
MOLECULAR AND EMPIRICAL
MOLECULAR AND EMPIRICAL
FORMULA OF A
FORMULA OF A
HYDRATE
HYDRATE
Section: 6.2
Section: 6.2
HYDRATES
•
Recall: Formula units (molecules) that
have a specific number of water
molecules attached
•
Ex: MgSO
4
• 7H
2
O
HYDRATES
HYDRATES
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
63.55 + 2(35.45) + 2(18.02)
= 170.49g/mol
MOLAR MASS OF HYDRATE
•
Find the M of anhydrous
CuCl
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 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
•
x
H
2
O
- NEED: # H
2
O in MgSO
4
•
x
H
2
O
-
Can calculate M of MgSO
4
= 120.38 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
120.38 g/mol
=
0.20269 mol
MgSO
4
RECALL:
RECALL:
n =
n =
m
m
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
=
1.42064 mol H
2
O
RECALL:
RECALL:
n =
n =
m
m
M
M
To find the formula of MgSO
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 MgSO
4
=
0.20269
= 1
0.20269
Moles of H
2
O =
1.42064
= 7
0.20269
1 mol MgSO
4
: 7 mol H
2
O
Therefore
the formula is MgSO
4
•7H
2
O
Homework
Homework
Read Section 6.2
Read Section 6.2
Hydrate Problems.
Hydrate Problems.
Hydrates are compounds with water molecules attached. Learn about hydrates, their chemical names, and how to calculate the percentage by mass of water in a hydrate. Understand the concept of anhydrous compounds and molar mass calculations for hydrates and anhydrous forms.
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Presentation Transcript
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 CaSO4 2H2O CaCl2 2H2O LiCl2 4H2O CHEMICAL NAME Calcium sulphate dihydrate Calcium chloride dihydrate 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 RECALL: n = m M = 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
To find the formula of MgSO4 xH2O Use RATIO: * RECALL: To find ratio and divide both mole values with it to find the ratio* Identify the lowest mole value Mole of MgSO4 = 0.20269 = 1 0.20269 Moles of H2O = 1.42064 0.20269 Therefore the formula is MgSO 4 7H2O = 7 1 mol MgSO4 : 7 mol H2O
Homework Read Section 6.2 Hydrate Problems.
