chemistry

An aqueous solution of Na2SO4 has a molal concentration of 0.293 mol kg-1. Its density is 1.0350 g mL-1.

Calculate the molar concentration of Na2SO4.
----- mol L-1

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

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  1. 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

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    posted by DrBob222
  2. 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 = ??

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    posted by DrBob222
  3. 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)

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    posted by DrBob222
  4. 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 :)

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

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    posted by mel<3
  6. sorry i mean mL woops :)

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    posted by mel<3
  7. 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

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  8. 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.

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    posted by rita
  9. woops i meant 0.005250707, still wrong.

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    posted by rita
  10. 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??

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    posted by Beth

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