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

Mandatory Experiment 4.4

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 + 2HCl  2NaCl + H2O + 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.

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

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

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.

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.

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

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

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.

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

Calculate the concentration of the sodium carbonate solution.

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

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?

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

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.

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.

Describe the physical appearance of hydrated sodium carbonate.

Explain water of crystallisation.

Name another compound that has water of crystallisation present.

Teacher Material

A fuller description of titration procedure is to be found in the Student Material relating to Mandatory Experiment 4.2.

The two common hydrated forms of sodium carbonate are Na2CO3.10H2O and Na2CO3.H2O. Washing soda is composed of large translucent crystals of

Na2CO3.10H2O. This material effloresces in air, becoming coated with a white powdery coating of Na2CO3.H2O.

Washing soda purchased in a ground down form is likely to have lost some of its water of crystallisation. A lower value than the expected value of 10 for x in Na2CO3.xH2O will therefore be obtained if this material is used.

Larger crystals of washing soda also tend to lose some of their water of crystallisation fairly readily, as does sodium carbonate decahydrate purchased from the usual suppliers.

Preparation of reagents

0.1 M hydrochloric acid solution:

Use ampoules of 1 M HCl and dilute 10 cm3 to 100 cm3 with deionised water.

Laboratory grade HCl is not sufficiently pure to be used as a primary standard; however dilute HCl can be titrated against sodium carbonate (see mandatory experiment 4.2).

Quantities per working group

100 cm3 0.1 M HCl

2 g hydrated sodium carbonate

5 cm3 methyl orange solution

Safety considerations

Safety glasses must be worn.

Chemical hazard notes

Sodium carbonate i: Hydrated sodium carbonate is irritating to the eyes and skin. Wear eye protection.

Solutions of methyl orange usually contain 50% ethanol and are flammable.

Disposal of wastes

Dilute with water, and flush to foul water drain.

Specimen Results (washing soda)

Mass of hydrated sodium carbonate = 1.57 g

Rough titre = 13.0 cm3

Second titre = 12.5 cm3

Third titre = 12.6 cm3

Average of accurate titres = 12.55 cm3

Volume of sodium carbonate solution used in each titration = 25.0 cm3

Concentration of hydrochloric acid solution = 0.1 M

Specimen Calculations

(a) First principles method

Volume of hydrochloric acid solution used = 12.55 cm3

Moles of hydrochloric acid used = 12.55 x 0.1 / 1000

= 0.001255 moles

Balanced equation:

Na2CO3 + 2HCl  2NaCl + H2O + CO2

1 mole 2 moles 2 moles 1 mole 1 mole

Moles of Na2CO3 used = 0.001255 / 2

= 0.0006275

Molar mass of Na2CO3 = 106 g mol-1

Mass of Na2CO3 present in 25 cm3 = 0.0006275 x 106 g

= 0.0665 g

Mass of Na2CO3 present in 250 cm3 = 0.665 g

Mass of Na2CO3.xH2O present in 250 cm3 =1.57 g

Mass of water present in this amount of Na2CO3.xH2O = 0.905 g

Percentage of water present in Na2CO3.xH2O = 0.905 X 100 / 1.57

= 57.64%

Moles of water present in this amount of Na2CO3.xH2O = 0.905 / 18

= 0.05028

Moles of Na2CO3 present in this amount of Na2CO3.xH2O = 0.0006275 x 10

= 0.006275

Value of x in Na2CO3.xH2O = 0.05028 / 0.006275

= 8.01

= 8

Formula of hydrated sodium carbonate: = Na2CO3.8H2O

(b) Formula method

VA x MA x nB = VB x MB x nA

12.55 x 0.1 x 1 = 25 x MB x 2

MB = 12.55 x 0.1 x 1 / (25 x 2) M

= 0.0251 M

Concentration of sodium carbonate solution = 0.0251 M

Moles of Na2CO3 present in 250 cm3 = 0.0251 / 4

= 0.006275

Molar mass of Na2CO3 = 106 g mol-1

Mass of Na2CO3 present in 250 cm3 = 0.665 g

Mass of Na2CO3.xH2O present in 250 cm3 = 1.57 g

Mass of water present in this amount of Na2CO3.xH2O = 0.905 g

Percentage of water present in Na2CO3.xH2O = 0.905 X 100 / 1.57

= 57.64%

Moles of water present in this amount of Na2CO3.xH2O = 0.905 / 18

= 0.05028

Moles of Na2CO3 present in this amount of Na2CO3.xH2O = 0.0006275 x 10

= 0.006275

Value of x in Na2CO3.xH2O = 0.05028 / 0.006275

= 8.01

= 8

Formula of hydrated sodium carbonate = Na2CO3.8H2O

Specimen Results (sodium carbonate monohydrate):

Mass of hydrated sodium carbonate = 1.51 g

Rough titre = 27.2 cm3

Second titre = 26.8 cm3

Third titre = 26.8 cm3

Average of accurate titres = 26.8 cm3

Volume of sodium carbonate solution used in each titration = 25.0 cm3

Concentration of hydrochloric acid solution = 0.091 M

Specimen Calculations

Using either the first principles method or the formula method, the percentage of water present in the sample of sodium carbonate monohydrate was found to be 14.37%.

The value of x in Na2CO3.xH2O was found to be = 0.0121 / 0.0122

= 0.99

= 1

Formula of sodium carbonate monohydrate = Na2CO3.H2O

Specimen Results (sodium carbonate decahydrate):

Mass of hydrated sodium carbonate = 1.60 g

Rough titre = 13.0 cm3

Second titre = 12.8 cm3

Third titre = 12.8 cm3

Average of accurate titres = 12.8 cm3

Volume of sodium carbonate solution used in each titration = 25.0 cm3

Concentration of hydrochloric acid solution = 0.091 M

Specimen Calculations

Using either the first principles method or the formula method, the percentage of water present in the sample of sodium carbonate decahydrate was found to be 61.44%.

The value of x in Na2CO3.xH2O was found to be = 0.055 / 0.0058

= 9.5

Formula of sodium carbonate monohydrate = Na2CO3.9.5H2O

Solutions to student questions

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?

It was stoppered, and then inverted several times. To ensure a homogeneous solution.

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

An indicator is a weak acid or a weak base. Use of an excessive amount of indicator will affect the titre value.

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.

(i) Pipette.

(ii) Burette.

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.

Volume of hydrochloric acid solution used = 10.6 cm3

Moles of hydrochloric acid used = 10.6 x 0.1 / 1000

= 0.00106

Balanced equation:

Na2CO3 + 2HCl  2NaCl + H2O + CO2

1 mole 2 moles 2 moles 1 mole 1 mole

Moles of Na2CO3 used = 0.00106 / 2

= 0.00053

Molar mass of Na2CO3 = 106 g mol-1

Mass of Na2CO3 present in 25 cm3 = 0.00053 x 106 g

= 0.05618 g

Mass of Na2CO3 present in 250 cm3 = 0.5618 g

Mass of Na2CO3.xH2O present in 250 cm3 = 1.51 g

Mass of water present in this amount of Na2CO3.xH2O = 0.9482 g

Percentage of water present in Na2CO3.xH2O = 62.79%

Moles of water present in this amount of Na2CO3.xH2O = 0.9482 / 18

= 0.05268

Moles of Na2CO3 present in this amount of Na2CO3.xH2O = 0.0053

Value of x in Na2CO3.xH2O = 0.05268 / 0.0053

= 9.94

= 10

Formula of hydrated sodium carbonate: = Na2CO3.10H2O

Describe the physical appearance of hydrated sodium carbonate.

White crystalline solid. However, if larger crystals are purchased, these tend to

have a clear glassy appearance.

Explain water of crystallisation.

Water chemically bound in the compound, which gives rise to the crystalline form or water present in definite proportions in crystalline compounds.

Name another compound that has water of crystallisation present.

Hydrated copper sulfate.