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Chemistry Help

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Exam Preparation [May. 31st, 2012|12:03 pm]
Chemistry Help

chemistryhelp

[alexidior]
Hi,

I have a Chemistry exam in 2 weeks and was hoping to get some help with questions I'm going through from previous years exams that were were given without answers and I'm stuck on.



I'm stuck on this one because despite having attended every class, I don't remember us ever having touched on calculating molar conductivity but am worried that they put E in there to trick us. The answer I'd go for if I saw this in my exam would prolly be A because it's the difference between CsNO3 and HF.



This I'm stuck on again because of option E. My understanding is that vapor pressure decreases after the addition of a solute so the highest would be pure water leaving me with C and D. I don't know where to go from here.

Thanks a million in advance.
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Comments:
[User Picture]From: smokeline
2012-05-31 04:11 am (UTC)
Not sure about the first question, but as for the second question...

You are correct that vapor pressure decreases after the addition of a solute; in particular, it decreases based on the number of moles of a solute. Since all the masses of the alcohols are the same, you are adding fewer moles of the heavier alcohol. You should be able to use that idea to rank them.
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[User Picture]From: babyfood22
2012-06-07 01:33 am (UTC)
For the first one, I would say that these are strong electrolytes, pretty much fully dissociated. This means you can approximate that they're additive, ie:

Conductivity = (nu_+ * lamda_+) + (nu_- * lamda_-)

That was awful. Sorry, no idea how to type in maths here. What I'm attempting to say is that lamda(CsF) = lamda(Cs) + lamda(F), and for all the others similarly. Therefore using this approximation, lamda(CsF) = lamda(CsNO3) - lamda(HNO3) + lamda(HF).

There's a different approximation for the limit of a weak electrolyte, in case you're interested. Ask your tutor/lecturer which you need to know or be able to derive - seriously, don't be scared to send them an email. Or just read up on it anyway if you think you have the time - more knowledge is never a bad thing when the exams are set right. Hope your exams go well!
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