I'm doing a high school chem project where I am making a calibration curve of copper(II) sulfate for spectrophotometry in which I make solutions with five different concentrations. The most concentrated one being what I though was 2 molal.

But I noticed that in that solution I could see specs of crystals floating around still after stirring for a while, and it did not want to dissolve completely.

It says on Wikipedia that copper(II) sulfate has a solubility of 1.26 molal (20 °C) in water. Does that mean that I can't have a solution with a concentration of 2 molal at that temperature?

  • 2
    $\begingroup$ Yes, it does... $\endgroup$ – Mithoron Jan 21 at 20:56

Yes, that's what it means. You could try to make a super-saturated solution, but that would mess up your experiment because some solute will eventually precipitate.

You don't want to have too high a concentration anyway because you might be out of the range of the instrument. It makes sense to have concentrations that the instrument can detect (not too low) and that don't max out the instrument (very concentrated solutions won't let any light through at the wavelength of interest).

You might also think about what the range of concentrations is for your unknowns. The calibration curve should cover that range nicely - calibration standards outside of that range don't contribute to your results much.


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