Determine volumetrically, the value of ‘n’ i.e., the number of water molecules of crystallisation in washing soda , 7.0 g of which have been dissolved per litre of the given solution. Provided
Indicator. Methyl orange.
End Point. Yellow to light pink (Acid in burette)
- Rinse and fill the burette with the standard HCl solution.
- Rinse the pipette with the sodium carbonate solution and pipette out 20 ml of this solution in the titration flask.
- Add 2-3 drops of methyl orange indicator to the titration flask. The colour of the solution becomes yellow.
- Note the initial reading of the burette and run acid solution slowly in the titration flask till the solution becomes light pink.
- Note the final reading of the burette and find out the volume of acid used.
- Repeat the procedure 4-5 times to get a set of at least three concordant readings.
Molarity of given HCl solution =
Volume of sodium carbonate solution taken for each titration = 20.0 ml
|S.No.||Initial reading of the burette||Final reading of the burette||Volume of the acid solution used|
Concordant volume = x ml (say).
The molarity of standard HCl = 0.1 M
Since in the balanced equation two moles of HC1 react with one mole of Na2C03.nH20.
But molar mass of hydrated sodium carbonate = (106 + 18n) g mol-1
Knowing the titre value, x, the value of n can be calculated.
The number of the molecules of water of crystallisation in washing soda is …..
Note. The result is expressed as nearest whole number. The value of n is 10 in this case.