Anonymous asked in Science & MathematicsChemistry · 1 month ago

Having trouble with this Chem problem?

 A solution was made by taking 33.88 g of an unknown ionic solid (i=3) and dissolving it in 175.0 g of water (Kf = 1.86 °C m-1; Kb = 0.512 oC m-1). The measured freezing point of the solution was 2.65 oC. Calculate the molecular weight of the unknown substance.

1 Answer

  • 1 month ago
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    Supposing the "measured freezing point" to have been really −2.65°C.  (Under no circumstances does adding a solute to pure water increase its freezing point, which is 0.00°C.)

    (0.00 - (-2.65))°C / (1.86 °C/ m) = 1.4247 m ions

    (1.4247 m) / 3 = 0.4749 m ionic solute

    (0.4749 mol/kg) x (0.1750 kg) = 0.0831075 mol solute

    (33.88 g) / (0.0831075 mol) = 407.7 g/mol

    [You didn't ask, but about the only compound I can think of with a molar mass like that and with i = 3 is calcium iodate monohydrate, Ca(IO3)2·H2O.]

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