[Ritik]

I am unable to find a difference between molarity and density of a substance. Suppose there is a 3M sodium thiosulphate solution. It has a density of 1.25 gm/ml. Since its a 3M solution,it means 3 moles are dissolved in every litre of solution which means 474 gm per litre of solution. But according to its density, 1250 gm should be dissolved in 1 L of solution. Why are these two quantities different?

[Dan]

That is not what density means. Density is total mass (of solute and solvent) divided by final volume.

1250 g NaS₂O₄ + 1 L water = a total mass of 2250 g

The final volume of this would also be >1 L.

[Borek]

Solution density tells you how much thiosulfate AND water is in a given volume.

1 mL @ 1.25 gm/mL means that 1 mL of the solution has a mass of 1.25 g, and that 1.25 g is a mixture of water and thiosulfate.

Actually 3 M thiosulfate solution has a denisty of 1.333 g/mL. 1 L contains 3 moles of the salt, or 3 mol×158.1 g/mol=474.3 g. 1 mL contains 0.474 g of the salt and 1.333-0.474=0.859 g of water.

Edit: Dan was slightly faster.

[Ritik]

Got it! Thanks!

[Hunter2]

1250 g NaS2O4 + 1 L water = a total mass of 2250 g

Sodium thiosulfate has the formula Na₂S₂O₃ x 3 H₂O as aqueous salt