Quantitative Chemistry

Question 1

What is the empirical formula?

Answer 1

The empirical formula of a compound is the simplest, whole number ratio of atoms of each element in a compound.

E.g. CH2

Question 2

What is the molecular formula?

Answer 2

The molecular formula shows the actual number of atoms of each element present in a compound.

E.g. C6H12

Question 3

What is water of crystallisation?

Answer 3

Water of crystallisation is water that is chemically bonded into a crystal structure.

Here is the formula for hydrated copper(II) sulfate. Notice that water of crystallisation is separated from the main formula by a dot.

CuSO4.5H2O ← water of crystallisation

Question 4

What does hydrated mean?

Answer 4

Hydrated means that the solid crystals contain water of crystallisation.

Question 5

What is an anhydrous substance?

Answer 5

an anhydrous substance contains no water of crystallisation.

Question 6

How can you remove water of crystallisation?

Answer 6

By heating it to constant mass, the water of crystallisation will evaporate.

Heating to constant mass involves heating the same for several minutes, weighing it and repeating this until two consecutive mass measurements are the same.

Question 7

How do you calculate x from the experiment data?

Answer 7

1. Calculate mass of the anhydrous salt (mass after heated - crucible mass)
2. Calculate moles of anhydrous salt (using moles = mass / RFM)
3. Calculate mass of the water of crystallisation lost (mass before heating - mass after heating to constant mass)
4. Calculate moles of water (using moles = mass / RFM)
5. Find x (by dividing moles by the smallest amount of moles)

Question 8

Find x in the formula: MgSO4.XH2O
Mass of empty crucible = 12.73g
Mass of crucible + hydrated magnesium sulfate = 13.96g
Mass after heating for 5 minutes = 13.56g
Mass after heating for 10 minutes = 13.33g
Mass after heating for 15 minutes = 13.33g

Answer 8

1. Mass of anhydrous salt = mass after heated - crucible mass = 13.33 - 12.73 = 0.6g
2. Moles of anhydrous salt = mass / RFM (of magnesium sulfate) = 0.6 / (24+32+16+16+16+16) = 0.005
3. Mass of the water of crystallisation lost = (mass before heating - mass after heating) = 13.96 - 13.33 = 0.63g
4. Moles of water = mass / RFM (of water)
   = 0.63 / (1+1+16) = 0.035
5. Find x (by dividing moles by the smallest number of moles)
   Smallest amount of moles (0.005 or 0.035) = 0.005
   Moles of anhydrous salt = 0.005 / 0.005 = 1
   Moles of water = 0.035 / 0.005 = 7