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Topic: chemistry - molarity, molality, and mole fraction  (Read 11038 times)

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abs

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chemistry - molarity, molality, and mole fraction
« on: August 02, 2005, 01:46:37 PM »
can anyone explain how to find the molarity, molality, and mole fraction when you are given the density and mass percent of solute?

example: Hydrochloric acid  density= 1.19 and mass percent of solute= 38
« Last Edit: August 02, 2005, 02:08:00 PM by geodome »
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Offline sdekivit

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Re:chemistry
« Reply #1 on: August 02, 2005, 02:05:08 PM »
molarity is given by:

mol solute / L solvent.

molality is given by:

mol solute / kg solvent

molfration is given by:

mol solute / (mol solute + mol solvent)

if you know the density of the solution (what units do you use?) you can calculate the mass of 1 L solution and then use the mass percentage to get the mass of HCl in 1 L. Now you can calculate the concentrations in the units above.

« Last Edit: August 02, 2005, 02:05:37 PM by sdekivit »
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Offline sdekivit

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Re:chemistry - molarity, molality, and mole fraction
« Reply #2 on: August 02, 2005, 02:18:10 PM »
here's a good site with questions to practice a little bit ( with tutorials):

http://lrc-srvr.mps.ohio-state.edu/under/chemed/qbank/quiz/bank9.htm
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Offline Borek

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Re:chemistry - molarity, molality, and mole fraction
« Reply #3 on: August 02, 2005, 03:24:27 PM »
Check concentration lectures and concentration questions, all definitions and some conversions are already covered

http:www.chembuddy.com/?left=concentration&right=toc

http://www.chembuddy.com/?left=concentration-questions&right=toc

although the questions section is rather small yet.
« Last Edit: August 02, 2005, 03:25:53 PM by Borek »
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abs

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Re:chemistry - molarity, molality, and mole fraction
« Reply #4 on: August 02, 2005, 05:32:57 PM »
I'm still not understanding. I went to one of those quizzes but they don't explain how to work out the problem.

example:The solubility of NH3 gas in water is 33.1% by weight and the density is 0.998 g/cm3. What is the molarity of a saturated solution of ammonia in water?
(Atomic weights: N = 14.01, H = 1.008, O = 16.00).

I know the answer is 19.4 because it told me but i don't know how to work out the problem. Can somone work out the problem so I can see how it is supposed to be done?
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Offline sdekivit

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Re:chemistry - molarity, molality, and mole fraction
« Reply #5 on: August 02, 2005, 05:42:22 PM »
take 1 L --> the mass is 998 g. There you need to take 33,1 % over to get the mass of NH3. Convert that to mol.

because the volume is 1 L you can say that the amount of mols you calculated is the molarity: mol / volume (L). then you'll indeed come to 19,4 M

( click on tutorial at the top of the page for an explanation)

« Last Edit: August 02, 2005, 05:48:49 PM by sdekivit »
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Offline Borek

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Re:chemistry - molarity, molality, and mole fraction
« Reply #6 on: August 02, 2005, 05:52:25 PM »
http://www.chembuddy.com/?left=concentration&right=percentage-to-molarity

Exact, step by step derivation of the equation for conversion between weight percentage and molarity.
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abs

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Re:chemistry - molarity, molality, and mole fraction
« Reply #7 on: August 02, 2005, 06:58:07 PM »
I think I got all that. The only thing that I'm not quite sure about is the mole fraction. Since you are only given hydrochloric acid for example and not something else that's its in how do you find the mole fraction?
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Offline Borek

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Re:chemistry - molarity, molality, and mole fraction
« Reply #8 on: August 02, 2005, 07:42:23 PM »
Quote from: abs on August 02, 2005, 06:58:07 PM
I think I got all that. The only thing that I'm not quite sure about is the mole fraction. Since you are only given hydrochloric acid for example and not something else that's its in how do you find the mole fraction?

You have two substances in the solution - hydrochloric acid (solute) and water (solvent). Calculate number of moles of both substances in some volume - for example in 1L - and use definition of molar fraction. That's all.
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