Molarity vs. Molality

[deleted357625]

Kaplan passage question asks whats the Ca concentration of 1 mole of CaCl2 to 1L of distilled water at 298K

A.) 1 M
B.) 1 m
C.) 2 M
D.) 2 m

i see the units as mole per Liter, and chose A as my answer for obvious reasons --> 1mol/1L = 1 M but the correct answer is molality 1 m
explain? It's been bugging me for a while


edit: should of put in Q&A --> forgot ;/

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[Zoopeda]

A most serendipitous post, as I'm currently in Gen Chem, and my final exam on this exact stuff is on Monday. It'd be great to get a few opinions, but it seems to me that's definitely an error in your book. I concur, the answer should be A) 1M. Molarity is, as you said, moles/liter, and molality is moles/kg(solvent). You would need to know the density of the solvent in order to calculate molality here, so getting an answer in "m" units is impossible. You could assume water as a density of 1g/ml, but I conclude Kaplan error.

 

[jevo]

Is that the actual question it asked? if there was mention of a change in temp, then I could see it being 1m since molality doesn't change with temp, whereas molarity does?

 

[deleted357625]

Explanation in book:

The molality of a solution is defined as the number of moles of solute added to 1 kilogram of solvent (you should know that one kilogram of water as a volume of one liter at room temperature.) Molarity is the number of moles of solute per liter of total solution. Since you are told that one mole of calcium chloride has been added to one liter of water, the total volume of the solution will be greater than one liter. Noticing this, you should know that molality is a far more convenient concentration unit. gay..I still don't think molality would be a great answer

horrible..

 

[psychkub]

That's a terrible question... I wouldn't worry about it. It sounds like you're well aware of the difference between molarity and molality.

...also, there's nothing "gay" about it.

 

[TriagePreMed]

Explanation in book:

The molality of a solution is defined as the number of moles of solute added to 1 kilogram of solvent (you should know that one kilogram of water as a volume of one liter at room temperature.) Molarity is the number of moles of solute per liter of total solution. Since you are told that one mole of calcium chloride has been added to one liter of water, the total volume of the solution will be greater than one liter. Noticing this, you should know that molality is a far more convenient concentration unit. gay..I still don't think molality would be a great answer

horrible..

That's a horrible question indeed, but you assume, as almost anyone would, that it is an ideal solution. However, since you had an answer in molality as your option, that one would be technically most correct.

...also, there's nothing "gay" about it.

I remember when I was in High School and that term was thrown around, but I agree with you entirely, at this stage you have to outgrow the childish words.

 

[fas376]

That's a horrible question indeed, but you assume, as almost anyone would, that it is an ideal solution. However, since you had an answer in molality as your option, that one would be technically most correct.


I remember when I was in High School and that term was thrown around, but I agree with you entirely, at this stage you have to outgrow the childish words.

It's not meant to be offensive, obviously, which is why I don't understand why people make a big deal out of it. My best friend is gay and agrees with me completely. Although I agree that we're getting old and should probably take this out of our vocabulary 😀

And as far as the question goes, I'm pretty sure it's 1 M. These review books aren't perfect, you know 🙂

 

[TriagePreMed]

It's not meant to be offensive, obviously, which is why I don't understand why people make a big deal out of it. My best friend is gay and agrees with me completely. Although I agree that we're getting old and should probably take this out of our vocabulary 😀

And as far as the question goes, I'm pretty sure it's 1 M. These review books aren't perfect, you know 🙂

I know it's not meant to be offensive. I am by no means am saying you're homophobic or something like that. It was the same way in high school for me. It's just a word that got misappropriated. However, it gives off this tacit approval that gay is negative, which most people that say it don't intend it, and once you get into the more and more adult world, you have to be much more careful about speech. My comment is more of a "hey man, just watch out with that language since it can be misinterpreted now that you're older" than jumping on you.

 

[SN2ed]

Thread closed. These types of questions belong in the Study Q&A forum.