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Topic: Molality of a Solution  (Read 8449 times)

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Offline Sis290025

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Molality of a Solution
« on: January 20, 2006, 03:43:48 PM »
A 0.420 M solution of a compound in water has a density of 1.390 g/mL at 20°C. What is the molality of this solution?

m = mol solute/kg solvent

Given: 0.420 mol of solute since we have 0.420 mol/1 L solution

So 1.390 g/mL * 1000 mL = 1390 g = 1.390 kg of solution.

But how do you find out the kg of solvent from here?
 
Please reply soon.
Thank you.
« Last Edit: January 20, 2006, 03:46:57 PM by Sis290025 »
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Offline Borek

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Re:Molality of a Solution
« Reply #1 on: January 20, 2006, 03:48:15 PM »
solution = solvent + solute
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Offline Sis290025

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Re:Molality of a Solution
« Reply #2 on: January 20, 2006, 03:54:02 PM »
Yes, I know that, but how am I to find the mass of the solute or solvent so I can subtract from the kg of solution?
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Offline Borek

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Re:Molality of a Solution
« Reply #3 on: January 20, 2006, 04:17:25 PM »
What is mass of solute in 1L of solution?
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Offline Sis290025

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Re:Molality of a Solution
« Reply #4 on: January 20, 2006, 05:46:53 PM »
How do you find that out without  the solute's molar mass?
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Offline Borek

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Re:Molality of a Solution
« Reply #5 on: January 20, 2006, 06:29:18 PM »
Oops, sorry - I have overlooked there is not molar mass given :(

I shouldn't answer any posts doing some stupid game betatests at the same time, the problem is - you have to make some money occasionally to survive ;)

So - no molar mass, no solution. I mean - you can prepare the solution, but you can't solve the question ;)

Molality = 1000 * Molarity / (1000 * Density - Molarity * SoluteMolarMass);
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Offline Sis290025

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Re:Molality of a Solution
« Reply #6 on: January 20, 2006, 07:43:05 PM »
So the answer is unreachable because the question is bogus?
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Offline Borek

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Re:Molality of a Solution
« Reply #7 on: January 20, 2006, 08:02:57 PM »
Seems so.
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Offline Winga

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Re:Molality of a Solution
« Reply #8 on: January 21, 2006, 03:21:05 AM »
Quote from: Sis290025 on January 20, 2006, 05:46:53 PM
How do you find that out without  the solute's molar mass?
Actually, you can find out the molar mass of a compound from dividing its density by its concentration, you will get "g/mol" as final unit after you modify it.

However, I am not sure that the given density belongs to the solution or solute.
« Last Edit: January 21, 2006, 03:27:37 AM by Winga »
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Offline Borek

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Re:Molality of a Solution
« Reply #9 on: January 21, 2006, 04:27:29 AM »
Quote from: Winga on January 21, 2006, 03:21:05 AM
Actually, you can find out the molar mass of a compound from dividing its density by its concentration

For a pure substance only. For example water - molar concentration about 54 mol/L, density 1000 g/L, molar mass 1000/54 = 18 g/mol.

Quote
you will get "g/mol" as final unit after you modify it.

Just because units are correct doesn't mean you calculated anything.

Quote
However, I am not sure that the given density belongs to the solution or solute.

To the solution - so not to the pure substance.
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