Molarity and Molality

[sugarbabee0]

What is the BEST explanation for the observation that 1.0 M HCl(aq) has a greater boiling point than 1.0 M HF(aq)?



A. HCl has a greater molecular mass than HF, so the molality of the HCl solution is greater than the molality of the HF solution.
B. HCl has a greater molecular mass than HF, so the molality of the HCl solution is less than the molality of the HF solution.
C. HCl does not dissociate completely in water, while HF does.
D. HF does not dissociate completely in water, while HCl does.

D is the best answer. Choices A and B can be eliminated immediately, because even though HCl has a greater molecular mass than HF, the solutions have the same molarity (which is based on moles of solute, not grams of solute). That means the two solutions have approximately the same molality.





If two solutions have the same molarity, then they have the same molality? Is that true in all cases?

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[collegestud2013]

Remember the formula for BP elevation is:

T = (Kb)(m)(i)

Obviously Kb is a constant and m like you said is the same for both HF and HCl, but i, the Van't Hoff factor, would be higher for HCl than HF. i = 2 is good approximation for HCl since it's a strong acid, but for HF it's btwn 1 and 2, with the exact number depending on both Ka for HF and the actual molarity/molality of the HF solution.

 

[sugarbabee0]

Remember the formula for BP elevation is:

T = (Kb)(m)(i)

Obviously Kb is a constant and m like you said is the same for both HF and HCl,

My question is why is m the same? One has a heavier mw than the other. Wouldn't that change the molality?
And does that mean any two species with the same Molarity have the same molality?

 

[Rabolisk]

Molality is moles of solute per liter of solvent.

 

[sugarbabee0]

Molality is moles of solute per liter of solvent.

Isn't it moles of solute per kg of solvent?

Does that mean that that the solute mass (kg) is not included in the denominator (kg of solvent)?

And if two different species with the same molarity are each mixed in identical solvents, do both solutions have the same molality?

 

[Rabolisk]

I do mean kg. Obviously I misspelled... Same molarity would mean same molality if the solutes did not add significant volume to the solution. You generally assume that with dilute solutions, unless told otherwise.