Determine the formula of hydrated Sodium Carbonate Essay

Na2CO3.x H2O is the formula for hydrated Sodium Carbonate, where x represents the number of water molecules present in the crystals.

The first method I will use to determine x, will involve heating the crystals. When the crystals are heated, they readily effloresce to form a white powder, the anhydrous form Na2CO3 1.

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Hydrated Sodium Carbonate

Spatula

Scales (accurate to 3 d.p.)

Bunsen burner

Crucible and lid

Pipe Clay Triangle

Tripod

Heatproof mat

Long handled tongs

Safety goggles

Tub and lid

Mass of crucible and lid (g)

Mass of crucible lid and crystals (g)

Mass of crucible, lid and anhydrous solid after 1st heating (g)

Mass of crucible, lid and anhydrous solid after 2nd heating (g)

A

B

C

D

Draw the above table

* Using accurate scales, record mass of crucible and lid to three decimal places in the table under A

* Approximately weigh 5.

000g hydrated Sodium Carbonate crystals into a tub

* Transfer the crystals from tub to empty crucible and weigh with lid

* Record mass under B

* Gently heat crucible and contents with lid on, for about 1 minute

* Remove lid and strongly heat for 4 minutes

* Using long handled tongs, remove crucible, replace lid and leave to cool, for 5 minutes

* Bunsen should be put onto yellow ‘safety’ flame

* Find mass of crucible, lid and Sodium Carbonate and record under C

* Strongly heat crucible with lid off for 2 minutes

* Using long handled tongs, remove crucible, replace lid and cool for 5 minutes

* Find mass of crucible, lid and Sodium Carbonate and record under D

To find the number of moles of H2O in the crystals, the following calculation should be used:

Mass of hydrated crystals

Eg

(B – A)

Mass of anhydrous solid

Fg

(D – A)

Mass of water of crystallisation

Gg

(D – B)

Moles of anhydrous sodium carbonate

Jmoles

Fg / Mr

Moles of water

Kmoles

Gg / Mr

Jmoles of Na2CO3 are combined with Kmoles of H2O and therefore, 1 mole of Na2CO3 will be combined with (J / K) moles of H2O.

The second method I will use to determine x, will involve a titration between hydrated sodium carbonate and hydrochloric acid.

Na2CO3 + 2HCl –> 2NaCl + CO2 + H2O

0.1M Hydrochloric acid

Hydrated Sodium Carbonate

Phenolphthalein Indicator

Distilled water

Burette (50 cm3)

Retort stand

Boss-head

Clamp

Funnel

Small tub and lid

Beakers (250 cm3)

Volumetric flask (100 cm3)

Conical flask (250 cm3)

Pipette (20 cm3)

Pipette filler

White tile

Safety goggles

Mass tub, lid and crystals (g)

Mass tub and lid (g)

Mass of crystals in solution (g)

Volume of HCl used in rough titration (cm3)

Volume of HCl used in 1st titration (cm3)

Volume of HCl used in 2nd titration (cm3)

Average volumes of both titers (cm3)

A

B

C

D

E

F

G

If I assume that x = 5, the molar mass of Na2CO3.xH2O would be 196gmol-1. A solution of 1M HCl should be used. The mole ratio states that, HCl: Na2CO3 = 2:1, therefore a standard solution of around 0.5M Na2CO3 should be used to give rational titres. The experiment is to be carried out a number of times; therefore a 100cm3 standard solution of 0.5M Na2CO3 will be adequate for 4 trials. Therefore the mass of hydrated Sodium Carbonate required for the titration will be:

0.5M x 18gmol-1 = 98g 98g x (100cm3 / 1000cm3) = 9.8g

Therefore 9.8g Na2CO3.xH2O will produce a standard solution of 0.5M, when added to 100cm3distilled water.

When pH is below 8.2, phenolphthalein indicator is colourless. When above pH 10.0, the indicator turns pink 2.

* Draw above table

Firstly a standard solution of hydrated Sodium Carbonate is required

  • Rinse volumetric flask (100 cm3) and funnel with distilled water
  • Approximately weigh 9.800g hydrated Sodium Carbonate crystals into small tub and replace lid
  • Using accurate scales, find mass of crystals and tub and lid, to three decimal places
  • Record mass in table under A
  • Place funnel in neck of volumetric flask and add crystals to flask
  • Rinse any crystals into flask with distilled water and make up to 100 cm3
  • Replace stopper on flask and shake to dissolve crystals
  • Find mass of empty tub and lid
  • Record this mass in the table, under B

The burette should be setup using a retort stand, boss head and clamp

  • Rinse all equipment with distilled water
  • Rinse burette with HCl
  • Rinse pipette with small amount of Sodium Carbonate solution
  • Fill up burette to 50 cm3 of HCl
  • Place white tile directly under the jet of burette and place conical flask on top of it
  • Using pipette, transfer exactly 20cm3 of Sodium Carbonate solution into conical flask
  • Place few drops of Phenolphthalein Indicator into conical flask
  • Open jet of burette and allow steady jet of HCl to enter flask, while swirling flask
  • As titration reaches end point, solution will turn from colourless to magenta
  • When end point is reached, record volume of HCl that was used, to 2 decimal places, under D
  • Empty conical flask and rinse with distilled water
  • Fill burette up to 50 cm3 with HCl
  • Using the pipette, place exactly 20cm3 of the sodium carbonate solution into conical flask
  • Place few drops of Phenolphthalein Indicator into conical flask
  • Open jet of burette and allow steady jet of HCl to enter the flask, while swirling flask
  • Allow approximately 80%D cm3 into flask
  • Allow 0.1cm3 HCl into flask and swirl, during one opening of jet
  • As titration reaches end point, allow one drop of HCl out of jet, and spray end of jet with distilled water
  • When end point is reached, it will be accurate to one drop of HCl
  • Record volume of HCl under E
  • Repeat above until two results within 0.1cm3 are obtained and record the 2nd volume of HCl under F

Safety goggles must be worn at all times during both experiments to ensure that no Sodium Carbonate or Hydrochloric acid comes into contact with the eyes. If there is contact, immediately rinse out eyes with water. If acid comes into contact with skin, rinse off immediately. 0.1M HCl will be used because a more concentrated solution may be hazardous. When bunsen’s are not being used, they should be placed on the yellow ‘safety’ flame, so they are visible. All mass that are recorded must be to 3 decimal places to ensure a high grade of accuracy. All volumes that are recorded must be to 2 decimal places.

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