Salts part 2
Starter
• How many words can you think of that start
an- ?
• What do they each mean?
Some definitions
• Anhydrous – literally means without water
• Hydrated – Hydration occurs when water
reacts with a compound producing a single
chemical product.
• Water of crystallisation – Many salts form dry
crystals containing molecules of water. This
water which does not wet the crystals is
known as water of crystallisation.
• Some compounds contain H2O in their structure. These compounds are called hydrates.
• This is different from (aq) because the H2O is
part of the molecule (not just surrounding it).
• The H2O can usually be removed if heated.
• A dot separates water: e.g. CuSO4•5H2O is
copper(II) sulfate pentahydrate.
• A greek prefix indicates the # of H2O groups.
What would you call this?
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SnCl2•2H2O –
tin(II) chloride dihydrate
1 - Na2SO4•10H2O
2 - NiSO4•6H2O
3 - sodium carbonate monohydrate
4 - barium chloride dihydrate
• 1 - sodium sulfate decahydrate
• 2 - nickel(II) sulfate hexahydrate
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3Na2CO3•H2O
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4BaCl2•2H2O
Calculating the water of crystallisation
• 1. Measure the mass of the salt you begin
with.
• 2. Heat the salt to make the water of
crystallization vaporize, leaving the
dehydrated salt.
• 3. Measure the mass of the salt you have left
after the heating.
• - The salt after heating is a dehydrated salt. You can
calculate exactly what amount of substance you have
with the help of the molar mass for the salt.
• - The mass lost during heating consisted of water of
crystallization. This can be used to calculate the
amount of water of crystallization contained in the salt.
• - If you divide the amount of water of crystallization
with the amount of dehydrated salt, you will know how
many units of water of crystallization exists per unit of
salt.
• Calculate the % of water in hydrated
magnesium sulphate MgSO4·7H2O
– Relative atomic masses: Mg = 24, S = 32, O = 16
and H = 1
– relative formula mass = 24 + 32 + (4 x 16) + [7 x (1
+ 1 + 16)] = 246
– 7 x 18 = 126 is the mass of water
– so % water = (126 / 246) x 100 = 51.2%
Some practice
• Q1 – A student found that a sample of hydrated
cobalt (II) chloride contained 5.2g of anhydrous
cobalt (II) chloride and 4.32g of water. Calculate
the formula of hydrated cobalt (II) chloride (Hint
cobalt chloride = CoCl2)
• Q2 – 13.9g of hydrated iron (II) sulphate crystals
were heated, driving off the water of
crystallisation, leaving 7.6 of anhydrous iron (II)
sulphate. Calculate the formula of hydrated iron
(II) sulphate. (Hint Iron sulphate = FeSO4)
• Q3 – Hydrated barium chloride is heated. Its mass
decreases from 30.5g to 26.0g. What is the
formula of the hydrated salt?
• Q4 – Hydrated sodium carbonate, Na2CO3·xH2O
was found to contain 62.9% water by mass.
Calculate the value of x
• Q5 – Potash alum has the formula
K2SO4·Al2(SO4)3·yH2O, calculate y, the number of
moles of water of crystallisation given that the
hydrated salt contains 45.57% water by mass
Answers
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Q1 – CoCl2·6H2O
Q2 – FeSO4·7H2O
Q3 – BaCl2·2H2O
Q4 – 10
Q5 - 24