Notes on waters of crystallization for Physical Sciences

1. QUANTITATIVE ASPECTS
OF CHEMICAL
CHANGE: WATERS OF
CRYSTALLIZATION
COMPILEDBY Ms. LQ RAMOTSABI

2. Waters of
crystallization
• Definition
• The water molecules
attached to an ionic crystal
compound.
• The ionic compound is
called a hydrate because it
contains water in it

3. • Thehydrate salt has the following chemical formula
ℎ𝑦𝑑𝑟𝑎𝑡𝑒 𝑠𝑎𝑙𝑡 ⋅ 𝑥𝐻2𝑂
e.g.𝐶𝑢𝑆𝑂4 ⋅ 10𝐻2𝑂
The𝐶𝑢𝑆𝑂4 is the ionic crystal compound
The10 represents the number of moles of water molecules that are
in the compound.

4. How to find the
amount of waters of
crystallization in a
compound?..
• We can findthe amount of waters of
crystallization byheating the crystal The
waters of crystallization will evaporateand
the crystal remains.
• Thereaction has the following equation:
• By weighing the starting and endproducts,
we can determine the amount of water that
was in the hydrate salt

5. Continued
A typical calculation problem will require you to find the number of
moles of the water molecules in the compound (the value of x)

6. LET’S DO AN EXAMPLE
Aluminiumtrichloride(𝑨𝒍𝑪𝒍𝟑)isan ionicsubstancethatformscrystalsinthesolidphase.Water
moleculesmay betrappedinsidethecrystallattice.Werepresentthisas: 𝑨𝒍𝑪𝒍𝟑⋅x𝑯𝟐𝑶.Carine
heatedsomealuminiumtrichloridecrystalsuntilallthewater hadevaporatedandfoundthatthemass
after heatingwas 2,8g. Themassbeforeheatingwas 5,08g. Whatisthenumberofmolesofwater
moleculesinthealuminiumtrichloridebeforeheating?

7. • Find the number of watermolecules
• Wefirst need to find n, the numberof water molecules that are present in the crystal. To do this wefirst note
that the mass of water lost is 5,08 g−2,8 g=2,28 g.
• Find the mass ratio
• Themass ratio is:
• 𝑨𝒍𝑪𝒍𝟑 :𝑯𝟐𝑶
2,8 : 2,28
• Find the mole ratio

8. • To workout the moleratio wedivide themass ratio by the molecular mass of each species:
• Next we convert the ratio to whole numbers by dividing both sides by the smaller amount:
𝑨𝒍𝑪𝒍𝟑:: 𝑯𝟐𝟎
𝟎.𝟎𝟐𝟎𝟗𝟗
𝟎.𝟎𝟐𝟎𝟗𝟗
:
𝟎.𝟏𝟐𝟔𝟔
𝟎.𝟎𝟐𝟎𝟗𝟗
1 : 6
Formula to find number of
moles: n=
𝒎
𝑴
𝑨𝒍𝑪𝒍𝟑 𝑯𝟐𝟎
n 𝟐. 𝟖
𝟏𝟑𝟑. 𝟑𝟓
𝟐. 𝟐𝟖
𝟏𝟖. 𝟎𝟎
n 0,02099 0,1266

9. continued
• The mole ratio of aluminium trichloride to water is 1:6
• And now we know that there are 6 moles of water molecules in the
crystal.
• The formula is 𝑨𝒍𝑪𝒍𝟑 ⋅ 𝒙𝑯𝟐𝑶
Also look at the example we did that is on the resource pack. It is also
about waters of crystallization but not number of moles, rather its
percent composition.

10. The end