prepwalk

1. CHEMISTRY
CALCULATIONS OF WATER OF
CRYSTALLISATION
Short revision series

2. AIM
 We are going to learn:
1. How to calculate percentage of water in
hydrated salts
2. Percentage of water of crystallisation
1. How to determine the formula of a salt
from the water of crystallisation

3. How to calculate percentage of water
in hydrated salts
 Question: Calculate the % of water in
hydrated MgSO4.5H2O salt crystals. (take
relative atomic masses of Mg = 24, S=32,
O=16, H = 1)
 SOLUTION:
 - First calculate the relative molecular
mass of MgSO4.5H2O = 24 + 32 + (4 x 16) +
(5 x (1 + 1 +16)
= 210

4. Solution cont.
Next, calculate the mass of 5H2O = 5 x 18 =
90
Therefore, the % of water is =
mass of water x 100
mass of MgSO4.5H2O
= 90/210 x 100
= 42.86%

5. How to calculate % of water of
crystallisation
 Question: Calculate the percentage of water of
crystallisation if 6.25g of CuSO4.xH2O (x
unknown) was heated gently until a mass of
4.00g is remaining. (take relative atomic masses
of Cu = 64, S=32, O=16, H = 1)
 SOLUTION:
 First calculate the mass difference = 6.25 – 4 =
2.25g (this is the mass of the water of
crystallisation)
 % of water of crystallisation = 2.25/6.25 x 100
= 36%

6. How to determine the formula of a
salt from the water of crystallisation
 Question: 6.25g of CuSO4.xH2O (x unknown)
was heated gently until a mass of 4.00g is
remaining. What is the formula of the salt?
(take relative atomic masses of Cu = 64,
S=32, O=16, H = 1)
 SOLUTION
After heating the mass of salt left is 4.00g,
while mass of water of crystallisation is mass
that was given off or lost, i.e 6.25 – 4 = 2.25g

7. SOLUTION CONT
The mass ratio CuSO4 and H2O = 4 : 2.25
Convert from mass ratio to mole ratio by
dividing the mass by their molecular mass of
each
Molecular mass of CuSO4 =160; while H2O=18
Mole ratio CuSO4 : H2O =4/160 : 2.25/18
= 0.025 : 0.125. *(divide by the smallest i.e
0.025) = 1 : 5
So the formula of the salt is CuSO4.5H2O