F.6/7 Chemistry Practical: A quantitative analysis of hydrated barium
chloride salt
Objective: Determination of number of molecules of water of crystallization in barium
chloride
Group size: Individual
Introduction
A quantity of barium chloride is accurately weighed, dissolved in de-ionized water
and made up to 250 cm3. The concentration of chloride ion in this solution is determined
by titrating with standard silver nitrate solution, using potassium chromate solution as
indicator. At the end point, the colour turns red. This indicates that silver chromate is
formed and most of the chloride ion is reacted to form white precipitate of silver chloride.
Theory: Give principle/reason to explain how do you determine the number of
molecules of water of crystallization in hydrated barium chloride salt. The following
questions should be useful to you.
1. What is the equation for the reaction between silver nitrate and barium chloride?
2. How can one determine the concentration of barium chloride if the concentration of
silver nitrate is known?
3. Could sodium chromate be used instead of potassium chromate in this experiment?
4. Could such a titration, using potassium chromate as indicator, be performed in
(a) in acidic solution
(b) in alkaline solution
5. Why is sodium sulphate solution added to the titration flask in step 5? (Hint: Is the
order of addition of sodium sulphate and potassium chromate important?)
6. Once the concentration of hydrated barium chloride is determined, how can you
determine the number of water molecule per one mole of the salt?
Chemicals:
2.5 g barium chloride, 10 cm3 of 0.2 M sodium sulphate solution, potassium
chromate indicator, 0.100 M silver nitrate standard solution
Additional apparatus: 50 cm3 burette, weighing bottle
Safety precaution
All chemicals are considered to be harmful. If silver nitrate solution is spurted onto your
skin, black spot will be left for several days.
Procedure
1. Weigh accurately between 2.3 and 2.5 g of barium chloride crystals into a clean, dry
100 cm3 beaker.
2. Add distilled water to dissolve the crystals and transfer the solution into a 250 cm3
volumetric flask.
3. Rinse the beaker with de-ionized water and transfer the rinsing liquid into the
volumetric flask. Add de-ionized water into the volumetric flask and make up to the
graduation line.
4. Mix the content of the flask well (Turn the flask upside down and up for at least 10
times) until the solution is homogeneous.
5. Pipette 25.0 cm3 of this solution into a conical flask, add approximately 10 cm3 of
F.6/7 Chem.Prac./Va11_BaCl2/p.1
0.2M sodium sulphate solution. Swirl the mixture.
6. Add 5 drops of potassium chromate solution as indicator and titrate against 0.10 M
silver nitrate solution until a permanent red colour is produced.
7. Repeat the titration to get two concordant readings.
Data and results
Mass of weighing bottle + barium chloride crystals =
Mass of weighing bottle =
Mass of barium chloride =
Titration data
Trial
3
Final burette reading (cm )
Initial burette reading (cm3)
Volume of silver nitrate used (cm3)
.
Mean titre =
Run 1
Run 2
Questions for calculation
1. What is the stoichiometric equation for barium chloride and silver nitrate?
2. How many moles of silver ions were added in the titration?
3. How many moles of chloride ions were there in the 25 cm3 aliquot?
4. How many moles of barium chloride in your weighed sample of barium chloride
crystals?
5. Calculate the number of water of crystallization in the crystal.
Questions for discussion
1. Comment the use of the standard solution, silver nitrate.
2. List possible errors in this experiment and estimate percentage error in your answer.
3. Can you apply this method to determine the concentration of a solution of hydrochloric
acid? If it is possible, give experimental procedure. If it is not possible, give reasons.
End
F.6/7 Chem.Prac./Va11_BaCl2/p.2