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I have pretty much zero experience in chemistry whatsoever but I wonder about how much crystal will I get from fully dissolved element when I reduce water temperature.

Variables

  • at $\mathrm{40^\circ C,\ 171}\pu{g}$ of $\ce{Pb(NO3)2}$ is fully dissolved in water

  • at $\mathrm{18^\circ C,\ }\ce{Pb(NO3)2}$ - How much is dissolved now? Everything that was not dissolved turned to crystals?

I know that it probably has something to do with this graph right here but I can't turn my mind to right direction. Can someone explain this problem and how can I find solution? enter image description here

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  • $\begingroup$ You need to know how much water is being used, not just the amount of the salt. $\endgroup$ Dec 13 '13 at 13:25
  • $\begingroup$ @NicolauSakerNeto Well since it's fully dissolved I calculated that it is dissolved in 244,28ml of water. $\endgroup$
    – sed
    Dec 13 '13 at 13:28
  • $\begingroup$ Being fully dissolved and being fully dissolved in the least amount of water are two different things. If you know the solution is saturated (i.e. it cannot dissolve any more of the salt), then you are correct. $\endgroup$ Dec 13 '13 at 13:32
  • $\begingroup$ @NicolauSakerNeto So what are next moves if we know that it is dissolved with the last amount of water possible? $\endgroup$
    – sed
    Dec 13 '13 at 13:41
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Assuming that the solution is saturated, meaning that if you add more salt to the solution it will not dissolve, and neglecting eventual evaporation of water, we have a mass of water of about 228g for 171g of salt (assuming solubility of 75g in 100g at 40°C).

Now, when cooling down to 18°C the solubility drops to ~55g / 100g water. This means that in our solution we still have $$228~\text{g}~\ce{H2O} \times \frac{55~\text{g}~\ce{Pb(NO3)2}}{100~\text{g}~\ce{H2O}}=125.4~\text{g}~\ce{Pb(NO3)2}\; . $$

The remaining 45.6g of the compound will have precipitated. Whether they form nice crystals or not depends on various other things, see this.

Now, if the solution is not saturated (meaning your salt can still be fully dissolved, but there may be room for more in the solution) the calculation is impossible if you don't know the mass of the water at the beginning.

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