Question
The hydroxides of the Group I metals are soluble in water. Most other metal hydroxides are insoluble in water.
(a) (i) Crystals of lithium chloride can be prepared from lithium hydroxide by titration.
25.0 cm3 of aqueous lithium hydroxide is pipetted into the conical flask.
A few drops of an indicator are added. Dilute hydrochloric acid is added slowly to the alkali until the indicator just changes colour. The volume of acid needed to neutralize the lithium hydroxide is noted.
A neutral solution of lithium chloride, which still contains the indicator, is left. Describe how you could obtain a neutral solution of lithium chloride which does not contain an indicator.[2]
(ii) You cannot prepare a neutral solution of magnesium chloride by the same method. Describe how you could prepare a neutral solution of magnesium chloride.[3]
(b) The concentration of the hydrochloric acid was 2.20 mol /dm3 . The volume of acid needed to neutralize the 25.0 cm3 of lithium hydroxide was 20.0 cm3. Calculate the concentration of the aqueous lithium hydroxide.[2]
LiOH + HCl → LiCl + H2O
(c) Lithium chloride forms three hydrates. They are LiCl.H2O, LiCl.2H2O and LiCl.3H2O.
Which one of these three hydrates contains 45.9% of water?
Show how you arrived at your answer.[3] [Total: 10]
Answer/Explanation
Ans:
(a) (i) add carbon / animal charcoal
filter
OR
repeat experiment without indicator
using same quantity / volume of acid
(ii) add magnesium metal / carbonate / oxide / hydroxide
to (hot) (hydrochloric) acid
cond: until in excess or no more dissolves or reacts
cond: filter (to remove unreacted solid)
(b) number of moles of HCl = 0.020 x 2.20 = 0.044
number of moles of LiOH = 0.044
concentration of LiOH = 0.044/0.025 = 1.769 (mol / dm3)
accept 1.75 to 1.77 need 2 dp
correct answer scores = 2
(c) (for LiCl.2H O)
mass of one mole = 78.5
percentage water = 36 / 78.5 x 100
45.9 so is LiCl.2H2O
only award the marks if you can follow the reasoning and it gives 45.9% of water
note: if correct option given mark this and ignore the rest of the response
allow: max 2 for applying a correct method to another hydrate, [1] for the method and [1] for the correct value, working essential