[mufan]

Hi,
I am struggling with the difference between solubility and concentration.  Sodium Bisulfite is sold as 38 to 40% sodium bisulfite in water.  but the solubility of it is 42 g / 100  ml.   So the maximum concentration should be 42 g / 142 grams of solution or 29.5%.  Am I missing something here?

thanks

[Burner]

Definition of concentration (from wikipedia):

concentration is the abundance of a constituent divided by the total volume of a mixture

So we don't divide the mass of solute by the mass of the whole mixture, but divide it by the total volume of the mixture instead.

[jeffmoonchop]

the solubility value is the concentration when the solution is saturated. they probably sell sodium bisulfite at slightly lower than the solubility value to prevent precipitation. If the concentration was higher than the solubility it is supersaturated and in order to return to equilibrium it will crystallise out until the liquid phase is at saturation again. not sure where you got 142g from. If the solubility is 42g/100ml the solution would be 42% if prepared to saturation.

[AWK]

Density of ~40 % bisulfite is ~1.31
Concentration is calculated on the pyrosulfite Na₂S₂O₅ content (pyrosulfite shows greater solubility).