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Topic: Relative molecular mass and mole  (Read 6611 times)

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Relative molecular mass and mole
« on: April 05, 2005, 01:14:29 PM »
i'm having some difficulty in solving a couple of numericals. they are:

1. What is the mole fraction of the solute in 2.5 M aqueous solution?
my doubt: to calculate mole fraction of the solute, isn't it essential to know the molarity of the solute? how do i get the molarity of the solute?

2. A solution is 0.15 mole fraction glucose (C6H12O6) and 0.850 mole fraction water. what is the molality of glucose in the solution?
my doubt: molality is no. of moles per 1 kg. no. of moles is weight/mass. how do i get molarity or no. of moles from the given data?

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Re:Relative molecular mass and mole
« Reply #1 on: April 05, 2005, 08:51:01 PM »
1. the molarity is given: 2.5M. the molarity is the number of moles of solute per unit volume of solution. You need to know the number of moles of solute present and number of moles of solvent present to work out the mole fraction. Molarity gives u the number of moles of solute.
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Re:Relative molecular mass and mole
« Reply #2 on: April 06, 2005, 06:33:06 AM »
thanks! and the second question?

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Re:Relative molecular mass and mole
« Reply #3 on: April 06, 2005, 07:19:15 AM »
0.15 mole of glucose is dissolved in 0.85 mole of water. The latter is easily converted to mass, so you will have 0.15 mole in known mass of solute. That's what molality is all about  ;)
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Re:Relative molecular mass and mole
« Reply #4 on: April 07, 2005, 10:01:46 AM »
0.15 and 0.85 are the mole fractions, not the molarity. how do i go abt it?

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Re:Relative molecular mass and mole
« Reply #5 on: April 07, 2005, 10:31:53 AM »
Remember the definitions of MolaRity and MolaLity.  Molarity is number of moles of solute per Liter of solution.  Molality is number of moles of solute per kg of solvent.  So to figure out the molality of a solution, you need the number of moles of solute and the mass of the solvent.  

You're given the mole fraction which states that in one unit of solution, you have 0.15 moles of glucose and 0.85 moles of water.  So in one unit of solution, you have 0.15 moles of glucose and 0.85 moles of water.  If you know the molar mass of water, you can calculate the mass of water in kilograms present in that one "unit" and then EASILY figure out the molality.   ;D
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Re:Relative molecular mass and mole
« Reply #6 on: April 11, 2005, 07:27:19 AM »
ah, ok! thanks!

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