Titration – Precipitation

Aim

The purpose of this experiment is to determine the number of molecules of water of crystallisation in hydrated barium chloride, i.e. to calculate the value of x in the formula BaCl₂•xH₂0.

Introduction

You titrate chloride ions with silver ions, according to the equation:

Ag⁺ _(aq) + Cl^– _(aq) → AgCl_(s)

This provides you with the data necessary to do the calculations. The indicator for the titration is potassium chromate(VI). When all the chloride ions have reacted, any more silver ions react with the indicator producing a red precipitate of silver chromate(VI). This is because silver chloride is less soluble than silver chromate(VI).

2Ag⁺ _(aq) + CrO₄^2- _(aq)  → Ag₂CrO_{4 (s)}

The end-point in this reaction is when one drop of aqueous silver ions produces a red tinge on the precipitate of silver chloride. Barium ions also react with chromate ions so the barium must be removed by adding sulphate ions:

Ba²⁺ _(aq) + SO₄^2- _(aq)  → BaSO_{4 (S)}

Note. This does not affect the concentration of chloride ions.

Requirements

• safety glasses
• weighing boat
• spatula
• barium chloride crystals (TOXIC)
• access to balance capable of weighing to 0.01 g
• beaker, 250 cm³
• wash-bottle of distilled water
• stirring rod
• volumetric flask, 250 cm³
• filter funnel
• dropping pipette
• burette, 50 cm³
• 2 beakers, 100 cm³
• silver nitrate solution, standardised (TOXIC & CORROSIVE)
• pipette, 10 cm³
• pipette filler
• conical flasks, 250 cm³
• sodium sulphate
• potassium chromate solution(TOXIC & OXIDISING)
• ‘silver residues’ bottle

Procedure

1. Prepare a standard solution of hydrated barium chloride by accurately weighing out between 1.4 g and 1.6 g of the salt. Dissolve this and make up to 250 cm³ in a volumetric flask. Record the exact mass in your copy of the table below.
2. Rinse the burette with some silver nitrate solution and fill. Don’t forget the tip.
3. Rinse the 10.0 cm³ pipette with barium chloride solution, and transfer 10.0 cm³ to a conical flask.
4. Add about 1 g of sodium sulphate crystals to the flask and swirl it.
5. Add 2-3 drops of potassium chromate(VI) indicator. Titrate the solution to the end-point, as shown by the first appearance of a permanent but faint reddish precipitate of silver chromate(VI). Use the first flask for a trial run. Enter your results in your copy of the results table.
6. Repeat steps 2 to 5 three times. Don’t wash the contents of the titration flasks down the sink – pour them into a ‘silver residues’ bottle.

Mass of bottle and contents before transfer, m1	g
Mass of bottle and contents after transfer, m2	g
Mass of sample, m = (m2-m1 )	g
Mass of BaCl2 • xH20 in 10.0 cm3	g

Table of Results (PDF)
Calculation

1. From the mean titre and concentration of silver nitrate, calculate the amount of chloride ions present in a 10.0 cm³
2. Calculate the mass of anhydrous barium chloride, BaCl₂ present in a sample.
3. Calculate the mass of water present by subtracting the mass of BaCl2 from the mass of BaCl₂xH₂0.
4. Determine the ratio of amount of BaCl₂ to amount of H₂0 and thus the value of x
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