Specification:
-The concept of amount of substance(moles) and its use to perform calculations
involving: masses of substances, empirical and molecular formulae, percentage composition, percentage yields, water of crystallisation
Water of Crystallisation is when water is present in the compound’s giant ionic lattice structure. The water crystallisation is represented with a dot in the formula. For example CuSO4.H2O. This compound is also called hydrous compounds as they contain water molecules in its structure. A compound that doesn’t have any water of crystallisation will be called an anhydrous compound. For example CuSO4.
N.B: Anhydrous compounds mean that the molecule does not contain any water molecules in its lattice.
Hydrated compounds are formed from anhydrous salt and water.
CuSO4 +H2O → CuSO4.H2O
Heating hydrated compounds:
When a hydrated compound is heated, it will form anhydrous salt and water.
Let’s look at an example:
CuSO4.5H2O → CuSO4 + 5H2O
As shown above, this will form an anhydrous salt (copper sulphate) and 5 water molecules. Sometimes you will need to balance the equation so check for this.
Now let’s look at an exam question:
13.2 g of a sample of zinc sulphate, ZnSO4.xH2O, was strongly heated until no further change in mass was recorded. On heating, all the water of crystallisation evaporated as follows: ZnSO4.xH2O → ZnSO4 + xH2O. Calculate the value of X in the zinc sulphate sample given that 7.4 g of solid remained after strong heating.
This is a very common question they can ask you!
So from this question, we know the compound and the mass we have left on the zinc sulfate but we want to know how much water there is represented by the x in the question.
Method 1:Using the moles to calculate the value of X
Step 1: Find the molar mass of the solid and water
First, you will need to find the number of moles of both water and zinc sulfate.
ZnSO4 is 65+32+ 64=161 ( the Mr of Zinc sulfate.)
H2O is 2+16=18 (Mr of water.)
Step 2: Calculate the no. of moles in Zinc sulfate and water
The moles of Zinc Sulfate is, therefore: 7.4/161= 0.045963 moles as the mass of the Zinc Sulfate is given in the question.
Therefore, the mass left of water is 13.2g-7.4g= 5.8g of anhydrous Zinc sulfate.
So, then we can find the number of moles in the water is 5.8/18= 0.3222 moles.
Step 3: Find the smallest common ratio
In this equation, we can divide the moles of water by the moles of zinc sulfate.
= 0.3222/0.045963 which gives 7 times.
So the equation is, therefore, ZnSO4.7H2O → ZnSO4 + 7H2O
Method 2: Using the Mr to calculate the value of X
Step 1: Find the number of moles in zinc sulfate
The moles of Zinc Sulfate is, therefore: 7.4/161= 0.045963 moles as the mass of the Zinc Sulfate is given in the question.
Step 2: Find the mr of ZnSO4.xH2O
As we know the moles of zinc sulfate we can find the mr of the whole compound as we know the ratio is a 1:1.
By doing mass of ZnSO4.xH2O the moles of zinc sulfate, we can find the mr.
13.2/0.045963=287.19 g/mol
Step 3: Generate equation by equating the Mr values
We know that 287.19 g/mol is equal to ZnSO4.xH2O so it will give an mr of 161+18x
Step 4: Rearrange to find the value of X
So, 287.19=161+18x
126.19=18x
x=7
Therefore, the formula is: ZnSO4.7H2O → ZnSO4 + 7H2O
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