Destroyer molality question

[lcataralc]

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Q: what mass of water is needed to dissolve 240g of NaCl to produce .20m aqueous solution.
A: 20kg

now correct me if I'm wrong, but since there are about 4 moles of NaCl, doesn't that mean that there are 8 moles of Na+ and Cl- ions, so since molality=solute mols/solvent mass, shouldn't the answer be 40kg?😕😕😕

 

[whatDATmean]

Yes, there are about 4.106 moles of NaCl.

Which means there are 4.106 of Na+ and 4.106 moles of Cl-.

So if you do 0.2m = 4.106 of NaCl / X kg of water

Solve for X.

You get roughly around 20 kg of water.

 

[justhanging]

Q: what mass of water is needed to dissolve 240g of NaCl to produce .20m aqueous solution.
A: 20kg

now correct me if I'm wrong, but since there are about 4 moles of NaCl, doesn't that mean that there are 8 moles of Na+ and Cl- ions, so since molality=solute mols/solvent mass, shouldn't the answer be 40kg?😕😕😕

I think your getting confused with that whole colligative property thing. Remeber molality is just mol/kg solution, it has nothing to do with that. Colligative properties only apply to osmotic pressure, freezing point depression, boiling point elevation, and vapor pressure depression.

 

[lcataralc]

Thanks for the help! After googling "colligative properties" I found out I was mixing up molality with colligative properties that use Van Hoff's factor (i) in boiling point elevation.