Question

The density of 3 molal sodium thiosulphate solution is 1.25g/cm^3 then mole fraction of Sodium thiosulphate

Answer 1

Mole fraction = Moles of substance/Total moles


1 mole of Na2S2O3 gives 2 moles of Na(+1) and 1 mole S2O3(-2)


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

Now,
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