The density of 3M aqueous solution of sodium thiosulphate is 1.25g/ml.Calculate the mole fraction of Na2S2O3?

Molecular weight of sodium thiosulphate penta-hydrate = 248 ;

1000 ml of 3M solution contains 248 X 3 g thiosulphate ;

1 ml of 3M solution contains (248 X 3)/1000 g or 0.744 g thiosulphate.

Amount of water per ml of aqueous thiosulphate solution

= 1.25 - 0.744 = 0.506 g .

Mole fraction of thiosulphate = (mole of thiosulphate)/(total number of moles)

= (0.744/248) / {(0.744/248) + (0.506/18)}

= 0.003/(0.003 + 0.02811)

= 0.0964 .

Molarity of Sodium & thiosulphate ions:

One molecule of thiosulphate produces 2 atoms of sodium ;

One mole of thiosulphate produces 2 moles of sodium ;

1000 ml of 3M thiosulphate will produce 2 X 3 gram-atoms or 6 gram-atoms of

sodium and 3 moles of thiosulphate ions.