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From 2013 ACS Local Exam:

We have a $\ce{KNO3}$ solution prepared in water, and we know the masses of solute and solvent as well as the molar masses. I know that we can easily find molality, but how come density and molarity cannot be determined?

Since the solvent is water, can't the mass be converted to volume to find both values?

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    $\begingroup$ Just how are you going to convert mass to volume, if you don't know density? $\endgroup$ – Ivan Neretin Mar 15 '16 at 5:58
  • $\begingroup$ As a side note in dilute solutions you could assume about 1.00 kg per L. There are lookup tables of course which you didn't have access. chemistry.mdma.ch/hiveboard/rhodium/pdf/chemical-data/… $\endgroup$ – MaxW Mar 15 '16 at 6:22
  • $\begingroup$ You know the volume of the original KNO3 and water for the pure substances, but there is a change in total volume when they are mixed to form a solution. $\endgroup$ – Chet Miller Mar 15 '16 at 12:27

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