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.

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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.

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Chemicals:

120 cm3 of standardized sodium hydroxide solution (about 0.2M), 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
Final burette reading/cm3
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:

  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.

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VA_06_NH4Cl /p.