Kaplan General Chem question, Boiling/Melting Point

[Toothguy80]

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25.5 g non volatile electrolyte placed in 500 g H2O. Boiling point is 101.56 degrees. What is the molecular weight of the solute? (Kb of H2O = 0.52 CKg/mol)

 

[Decan]

25.5 g non volatile electrolyte placed in 500 g H2O. Boiling point is 101.56 degrees. What is the molecular weight of the solute? (Kb of H2O = 0.52 CKg/mol)

First, you have to know that Tb = mKb where Tb is boiling point elevation and m is molality. So set up your equation to figure out molality m = 1.56/.52

Molality is mol solute/kg solvent, so solve for mol solute by multiplying molality by kg solvent (.5kg). Finally, divide the 25.5 g solute/mol solute to get your molecular wt.

 

[mhamilton23]

did it mention whether it was a weak or strong electrolyte? If it was weak enough, we can assume that it will behave as if it's a non-dissociating molecule (for example, glucose) and the molecular weight can be calculated by the way Decan described. Otherwise, if it's a strong electrolyte, then we'll have to take into account the van't hoff factor (i), where i >= 2 for a strong electrolyte. If we don't account for "i", the # of moles calculated will be (i)(true # of moles of solid), and therefore our calculated molecular weight will be (true molecular weight of solid/i).

I think this is correct. Feel free to correct me if this is not true, I don't want to mislead anyone.

Thanks

 

[Decan]

did it mention whether it was a weak or strong electrolyte? If it was weak enough, we can assume that it will behave as if it's a non-dissociating molecule (for example, glucose) and the molecular weight can be calculated by the way Decan described. Otherwise, if it's a strong electrolyte, then we'll have to take into account the van't hoff factor (i), where i >= 2 for a strong electrolyte. If we don't account for "i", the # of moles calculated will be (i)(true # of moles of solid), and therefore our calculated molecular weight will be (true molecular weight of solid/i).

I think this is correct. Feel free to correct me if this is not true, I don't want to mislead anyone.

Thanks

The problem states that the electrolyte is non-volatile so it will not dissociate in solution. For this reason, i = 1.

 

[mhamilton23]

Volatility is a measure of a substance's tendacy to enter the gas phase, not its ability to ionize. Since the question stated it was a nonvolatile electrolyte, we know that it will dissociate, but will not vaporize and supress the solvent's vapor pressure by reducing the mole fraction of the solvent in the gas phase. The extent of dissociation depends on the strength of the electrolyte. If it doesn't ionize, then the question should have stated "a nonvolatile, nonelectrolyte was placed..."

What do you guys think?
Thanks