[afchick7689]

Ok the question is as follows:

If 100 mL of water are evaporated from a 0.086 molal solution of sodium chloride, what is the resulting molality?  Assume the original is in 1 kg solvent.  

--So i thought to convert the 100 mL of water to moles because molality needs moles of the solute, but then when i plugged this into the molality equation, it didnt work out to be 0.086m like it should be? What am i doing wrong?

m=moles of solute/kg of solvent
Density of Water = 1g/mL
Molar mass of water = 16.02 g/mol
100 ml H2O|1 g H2O |1 mol H2O = 5.5494 mol H2O
                |1 mL H2O |18.02g H2O

0.086 m = 5.5494 moles H2O/ 1 kg NaCl --??

And then if i made x the original moles of water, and then x-100 the situation from the problem, it looks like this:
 m = X - 5.5494 moles H2O/ 1 kg NaCl
0.086 m = x moles H2O / 1 kg NaCl  --which again doesnt work out because this says im taking away more moles of water than i started with..


 

[Borek]

0.086 m = 5.5494 moles H2O/ 1 kg NaCl --??

Seems to me you have mixed solvent and solute at this point. 0.086 molal NaCl contains 5g of NaCl in 1000g of solution.

CASC rules

[afchick7689]

ooo , you're right, thanks... but then i have another question-- how do I subtract the 100 mL from 1000grams?

-thankss

[afchick7689]

actually i just answered my own question--

thanks so much for the *delete me*!