Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Sis290025 on January 20, 2006, 03:43:48 PM
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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.
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solution = solvent + solute
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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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What is mass of solute in 1L of solution?
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How do you find that out without the solute's molar mass?
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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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So the answer is unreachable because the question is bogus?
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Seems so.
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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.
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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.
you will get "g/mol" as final unit after you modify it.
Just because units are correct doesn't mean you calculated anything.
However, I am not sure that the given density belongs to the solution or solute.
To the solution - so not to the pure substance.