The density of a 3M sodium thiosulphate solution is 1.25 gm cm. Calcul

1.	The density of a 3M sodium thiosulphate solution is 1.25 gm cm. Calculate themolalities of Na" and S O ions; and mole fraction of sodium thiosulphate.
Answer» (i) Mole fraction = Moles of substance/Total moles (ii) 1 mole of Na2S2O3gives 2 moles of Na+and 1 mole S2O32- Molecular wt. of sodium thiosulphate solution (Na2S2O3) = 23 * 2 + 32 * 2 + 16 * 3 = 158 (ii) Mass of 1 litre solution = 1.25 * 1000 g = 1250 g [∵ density = 1.25g/l] Mole fraction of Na2S2O3 = Number of moles of Na2S2O3/Total number of moles Moles of water = 1250 – 158 3/18 = 43.1 Mole fraction of Na2S2O3= 3/3 + 43.1 = 0.065 (iii) 1 mole of sodium thiosulphate (Na2S2O3) yields 2 moles of Na+and 1 mole of S2O2-3 Molality of Na2S2O3= 3 1000/776 = 3.87 Molality of Na+= 3.87 * 2 =7.74m Molality of S2O2-3=3.87m

The density of a 3M sodium thiosulphate solution is 1.25 gm cm. Calculate themolalities of Na" and S O ions; and mole fraction of sodium thiosulphate.

Answer»

(i) Mole fraction = Moles of substance/Total moles

(ii) 1 mole of Na2S2O3gives 2 moles of Na+and 1 mole S2O32-

Molecular wt. of sodium thiosulphate solution (Na2S2O3) = 23 * 2 + 32 * 2 + 16 * 3 = 158

(ii) Mass of 1 litre solution = 1.25 * 1000 g = 1250 g

[∵ density = 1.25g/l]

Mole fraction of Na2S2O3

= Number of moles of Na2S2O3/Total number of moles

Moles of water = 1250 – 158 *3/18 = 43.1

Mole fraction of Na2S2O3= 3/3 + 43.1 = 0.065

(iii) 1 mole of sodium thiosulphate (Na2S2O3) yields 2 moles

of Na+and 1 mole of S2O2-3

Molality of Na2S2O3= 3 * 1000/776 = 3.87

Molality of Na+= 3.87 * 2 =7.74m

Molality of S2O2-3=3.87m