[The Guy]

I have to make a %15 solution of Na2S2O3, but I only have Na2S2O3.5H2O

I had hard time figure out how to make that solution from the hydrated form

Could you please help me on this? and if you know any good refference for such calculation? I need to review these things and build up my skills.

Thank you

[ramboacid]

The ratio of the mass of the sodium thiosulfate to the mass of the water is constant per unit of hydrate. Thus, when you add x grams of hydrate, a proportion p of the mass will be the salt and the proportion of water in the hydrate will be  (1-p). You'll have to take into account the added water from the hydrate because your solution will be based on percent mass.

[Arkcon]

You can consider the hydrate's water an impurity in your solid.   What mass of solid would you use to make the solution if you had the anhydrous form?  You need more, because some of you solid's mass is water.  But you know how much more, because you have the formula, you know how much "false" mass the hydrate solid has.

[The Guy]

  I really appreciate your comments guys

Smart logic