chemistry
posted by write2khin on .
An aqueous solution of Na2SO4 has a molal concentration of 0.293 mol kg1. Its density is 1.0350 g mL1.
Calculate the molar concentration of Na2SO4.
 mol L1
Calculate the mass percent of Na2SO4 in the solution.
 %
What is the mole fraction of Na2SO4 in the solution?

How to work out , i cant figure it out

0.293 m = 0.293 moles/kg solvent.
Take enough to have 0.293 mole which will have 1 kg solvent.
0.293 moles x molar mass Na2SO4 = about 41 g (but you need to do it exactly). The solution thus has a mass of 1000 + 41 = 1041. Use the density to convert to mL, then to L and moles/L = M 
0.293 m = 0.293 moles Na2SO4/1000 g solvent.
0.293 x 142.043 = about 42 grams solute (you need to do it accurately). The solution has a mass of 41+1000 = about 1041 grams. mass percent = (mass solute/g soln)*100 = (41/1041)*100 = ?? 
0.293 m = 0.293 moles/kg solvent.
Take enough solution to have 0.293 moles which will be 1 kg solvent.
0.293 moles Na2SO4.
moles H2O = 1000/18.015 = about 55.
Total moles = 55+0.293.
mole fraction Na2SO4 = (moles Na2SO4/total moles) 
thanks dr bob... this really helped :) but in question 1 & 2, why did u add 41 to 1000? where did u get the 1000 from?! thanks :)

does anyone know how to convert to Ml in question one using the density?? im really confused :/

sorry i mean mL woops :)

Question one answer, extended explanation :
divide 1041 g by the density (1.0350) to get volume 1,006.382609mL
then use formula c = n/v
c = 41.606 / (142 x 1,006.382609)
= 0.000291141 mol/mL x 1000
= 0.291141756 mol/L 
DrBob's solution for the molar fraction is wrong. I did what he said moles Na2SO4/ total moles: 0.293/55.802.... to get 0.00250679 and in my online quiz it was wrong.

woops i meant 0.005250707, still wrong.

I don't understand how we can know the total moles of solution if the question doesn't specify what the aqueous solution is. Do we always assume its water??