F.6/7 Chemistry Practical: Solubility of Ammonium Chloride
Objective: To determine the solubility of ammonium chloride by the method of back titration
Group size: Individual
Introduction
This experiment illustrates the use of the technique of back-titration which may be used to
determine the number of moles of a substance present in a sample when a direct titration is
not possible. The procedure is to add to the substance a known excess of a reagent which
reacts with it (albeit slowly) and can be titrated against a third substance to determine how
much remains in excess and thus, by difference, how much reacted.
----------------------------------------------------------------------------------------------------Theory: Give principle/reason to explain how do you determine the solubility of
ammonium chloride. The following questions should be useful to you.
1. Give details (not experimental steps), to illustrate the technique of 'back-titration' for the
determination of solubility of ammonium chloride.
2. Give an equation for the reaction in step 3.
3. Describe how do you carry out the practical: "…until no more ammonia evolved" in step
3.
4. Give an equation for the reaction in step 4.
5. Suppose the mean titre of your titrations is V, express the solubility of ammonium
chloride in terms of V.
----------------------------------------------------------------------------------------------------Chemicals:
120 cm3 of standardized sodium hydroxide solution (about 0.2 M), 120 cm3 standardized
hydrochloric acid solution (about 0.1 M), 5 cm3 of saturated ammonium chloride solution,
previously filtered, methyl orange indicator, red litmus paper.
Additional apparatus: 5.0 cm3 pipette, 2 pieces of 25.0 cm3 burette, 50 cm3 conical flask.
Procedure
1. Pipette 5.0 cm3 of the saturated ammonium chloride solution into a 500 cm3 conical flask.
Add 245 cm3 of distilled water into this flask and shake well.
2. Pipette 25.0 cm3 portions of this solution into each of three 250 cm3 conical flasks.
3. Add 25.0 cm3 of 0.2 M sodium hydroxide solution to each of the conical flasks and heat
until the solutions start to boil. Adjust the flame so that the solutions boil gently and
continue heating until ammonia is no longer evolved (Before experiment, you should
devise a method to carry out this step.)
4. Cool each flask and titrate the contents against the standardized hydrochloric acid using
methyl orange. Try to get two concordant readings. Record the results in tabular form.
Data and Results
Concentration of sodium hydroxide solution = __________
Concentration of hydrochloric acid = ____________
Colour change of the indicator before and after the end point =_____________
Titration Data
Run
1
2
3
3
Final burette reading/cm
Initial burette reading/cm3
Volume of HCl used/cm3
Average titrant:
Calculation: (The following questions help you to calculate the solubility of NH4Cl)
1. How many moles of sodium hydroxide are present in 25.0 cm3 of 0.20 M sodium
hydroxide solution?
2. How many moles of hydrochloric acid were added in your titration?
3. How many moles of sodium hydroxide reacted with 25.0 cm3 of ammonium chloride
solution?
4. How many moles of ammonium chloride were present in 25.0 cm3 of the diluted solution?
5. How many moles of ammonium chloride were present in 5.0 cm3 of the saturated
solution?
6. What is the solubility of ammonium chloride in water at room temperature in mol dm-3?
Discussion:
VA_06_NH4Cl /p.1
1. Why can the solubility of ammonium chloride not be determined by evaporating a
saturated solution to dryness and weighing?
2. What is the purpose of boiling the mixture in step 3? Explain.
3. Give safety precautions for this experiment.
4. Give error sources and estimate percentage error for your solubility of ammonium chloride.
Hand in your report to Mr. Lam before 4:30 p.m.
VA_06_NH4Cl /p.2