Determination of the Amount of Water of Crystallisation in Hydrated Sodium Carbonate

Determination of the amount of water of crystallisation in hydrated sodium carbonate

Student Material

Theory

Hydrated sodium carbonate has the formula Na2CO3.xH2O, where x is the number of molecules of water of crystallisation present. In this experiment, x is determined by titration of a solution made using hydrated sodium carbonate with a standard solution of hydrochloric acid. The equation for the reaction is

Na2CO3 2NaCl + H+ 2HCl 2O + CO2

Methyl orange indicator solution is used. At the end-point the indicator changes colour from yellow to peach/pink.

Chemicals and Apparatus

0.l M hydrochloric acid

Hydrated sodium carbonate i

Methyl orange indicator

Deionised (or distilled) water

Clock glass

Stirring rod

Burette (50 cm3)

Retort stand

Boss-head

Clamp

Filter funnel

Beakers (250 cm3)

Volumetric flask (250 cm3)

Conical flask (250 cm3)

Pipette (25 cm3)

Pipette filler

Dropping pipette

Wash bottle

White card

White tile

Safety glasses

Procedure

NB: Wear your safety glasses.

1.

Weigh accurately approximately 1.5 g of hydrated sodium carbonate into a beaker.

2.

Add about 50 cm3 of deionised water and stir to dissolve the sample.

3.

Transfer all of the solution into a 250 cm3 volumetric flask. Rinse the beaker with deionised water and add the washings to the volumetric flask.

4.

Make up the volumetric flask to the mark, using a dropper to add the final amounts of deionised water. Stopper the flask and invert several times.

5.

Rinse the burette, pipette and conical flask respectively with deionised water.

6.

Rinse the burette with hydrochloric acid solution, and rinse the pipette with sodium carbonate solution.

7.

Titrate the sodium carbonate solution against 0.1 M hydrochloric acid, using 25 cm3 of sodium carbonate solution in the conical flask and methyl orange as indicator.

8.

Repeat the titrations until the readings agree to within 0.1 cm3.

9.

Calculate the concentration of the sodium carbonate solution.

10.

Determine the formula of hydrated sodium carbonate.

Table of Results

Mass of hydrated sodium carbonate =

Rough titre =

Second titre =

Third titre =

Average of accurate titres =

Volume of sodium carbonate solution used in each titration =

Concentration of hydrochloric acid solution =

Concentration of sodium carbonate solution =

Moles of Na2CO3 in 25 cm3 of solution =

Molar mass of Na2CO3 =

Mass of Na2CO3 present in 25 cm3 =

Mass of Na2CO3 present in 250 cm3 =

Mass of Na2CO3.xH2O present in 250 cm3 =

Mass of water present in this amount of Na2CO3.xH2O =

Percentage of water present in Na2CO3.xH2O =

Moles of water present in this amount of Na2CO3.xH2O =

Formula of hydrated sodium carbonate =

^ Questions relating to the experiment

1.

What was done to the volumetric flask and its contents immediately after the solution had been made up to the mark with deionised water? Why was it important to do this?

2.

In acid-base titrations it is preferable to use as little of the indicator as possible. What is the reason for this?

3.

Give the name of a suitable piece of apparatus to measure accurately (i) the 25cm3

portions of sodium carbonate solution, (ii) the volume of hydrochloric acid

needed for a complete reaction.

4.

In a similar experiment, 1.51 g of hydrated sodium carbonate was used. If the average titre reading was 10.6 cm3, calculate the value of x in Na2CO3.xH2O.

5.

Describe the physical appearance of hydrated sodium carbonate.

6.

Explain