Molarity/molality

A 3.29 m solution of the solute H2O dissolved in the solvent acetone is available. Calculate the mass ( kg ) of the solution that must be taken to obtain 26.70 mol of H2O.

m= molality

I have no idea how to approach this problem, I'm not interesting in getting the answer, I actually what to learn how to do it.
Thanks =D

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  1. molality= molessolute/kgsolvent

    3.29=26.70molH20/kg solvent

    solve for kg solvent. Then add the mass equivalent of 26.70mole water to get kg of solution.

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    bobpursley
  2. I found another post on this site, which explains a similar problem.
    :)

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  3. Thank you!
    I got the correct answer.

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  4. Here is what I would do but check my thinking.
    26.70 moles = 26.70 x 18.015 g H2O/mol = 59.269 g H2O (I know that's too many significant figures but I keep them in my calculator and round at the end).
    So the SOLUTION has a mass of 59.269 g + 1000 g = 1059.269 g. Thus, 1000 g of that solution contains how many g H2O?
    59.269 g H2O x (1000/1059.269) = 55.953 g H2O which is 3.105 moles in the 1000 g SOLUTION. We want 26.7 moles.
    1000 g x (26.70 moles/3.105 moles) = 8599.03 g solution.
    Now we must determine how much of that 8599.03 g is solute and how much solvent. That should do it.
    I like to take this solution backwards now and see if it is 3.29 m. SOOO,
    26.70 moles H2O x (18.015 g/mol) = 481 g H2O. Total solution is 8599.03 and -481 = 8118.03 g acetone + 481 g H2O
    Therefore, m = moles/kg solvent = 26.70/8.118.03 = 3.289 which I would round to 3.29. Again, check my thinking.

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  5. This makes more sense, but your first step is a little confusing. when I do 26.70 x 18.015 g H2O/mol I get 481.005

    but when I do:
    3.29 m x 18.015 g H2O/mol then I get your ans: 50.269 g H20

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  6. It bothered me that the answer by Bob Pursley didn't agree better with my answer. The reason is that the calculation I did of
    1000g x (26.70/3.1059) = 8596.54 but I typed in 3.105 which is the difference.

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  7. I just dropped the last 5.

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  8. And I'm a little unsure as to why after you find the mass of the solution 1059.269 you find how many g of H20 are in 1000 g of that soln. Why can't we just use the 1058.269 g? Was it to get the perfect 1000g (1kg) or was that unnecessary?

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  9. In regards to your latest post, I meant the very first step, not the step you did to check the answer. I'm assuming the first line is a typo?

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  10. I'm not sure of the question but if I interpret it correctly, you would simply subtract 1059.269-59.269 to get 1000 g H2O but we know that. What the problem asks us to do is to take 1000 g of the SOLUTION which will actually contain 55.953 g H2O (not 59.269g) and the difference (944.047 g)acetone solvent (not 1000 g).

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  11. Ohhhhh
    That makes so much more sense. I didn't fully understand what the problem was asking.

    Thank you so very much =)

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  12. I did make a typo in this last answer and I've corrected it below.
    I'm not sure of the question but if I interpret it correctly, you would simply subtract 1059.269-59.269 to get 1000 g H2O(Should have said 1000 g acetone) but we know that. What the problem asks us to do is to take 1000 g of the SOLUTION which will actually contain 55.953 g H2O (not 59.269g) and the difference (944.047 g)acetone solvent (not 1000 g).

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