What is the molar mass of unknown 1?

What is the concentration m, (mol solute/kg solvent), when unknown 1 has a normalized freezing point of -1.6 degrees Celsius?

When Water has a Kf factor of +1.86 degree Celsius/m

And unknown 1 has a Hoff factor of 2 since Unknown 1 is either NaCl or KI.

So again what is the concentration m, Molality?

What is the moles?

What a the molar mass(g/mol)? Assuming we used 1.5 g of Unknown 1.

Update:

Also some other values: 

Volume of Sample 20 mLMass of Solid, experimental to help answer “what is the moles” 1.518 g = 0.001518 kg.

1 Answer

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  • 1 month ago

    "So again what is the concentration m, Molality?"

    (0.0 - (-1.6))°C / (1.86°C/m) = 0.8602 m = 0.86 m ions

    (0.8602 m) / 2 = 0.4301 m = 0.43 m salt

    "What is the moles?"

    It depends on the mass of solvent, which was not given.  So this question cannot be answered.

    "What a the molar mass(g/mol)?"

    Divide 1.5 g by the number of moles determined in the previous question.

    - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

    If it is known that unknown 1 is either NaCl or KI, the mass of solvent must be either 

    (1.5 g) / (58.4430 g NaCl/mol) / (0.4301 mol/kg) = 0.0597 kg = 59.7 g

    or

    (1.5 g) / (166.00277 g KI/mol) / (0.4301 mol/kg) = 0.0210 kg = 21.0 g

    Since this is apparently a school problem, I'd bet on the second possibility which is a rounder number, so as a real guess, I'd say unknown 1 is KI, which has a molar mass of 166 g/mol.

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