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Find the molality

  1. Feb 13, 2009 #1
    1. The problem statement, all variables and given/known data
    Find the molality of a sulfuric acid solution which contains 27,0% by mass of H2SO4 and a density of 1,198 g/mL

    2. Relevant equations
    molality = moles of solvent / mass of H2SO4 (kg)

    3. The attempt at a solution
    If we have 1kg of the solution, then we have 0,270 kg of H2SO4
    We also have 1 - 0,270 = 0,730 kg of water

    If one mole of water weighs 18 grams and we have 730 grams of water, then we have (730/18) moles of water.

    molality = moles of solvent / mass of H2SO4 (kg)
    = (730/18) moles / 0,270 kg of H2SO4
    = 150,2 moles/kg

    This is not the correct answer, the correct answer is 3,78
    What to do??
    Thank you
  2. jcsd
  3. Feb 13, 2009 #2


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    Homework Helper

    I'm no chemist, but I think molality is moles of solute per kg of solvent. Look this up in your textbook to make sure you are starting with the right definitions.
  4. Feb 13, 2009 #3
    I see...your problem in the equation for molality. Molality is equal to moles of solute over one kilogram of solvent. I think you flipped your equation
  5. Feb 13, 2009 #4
    my definition of moles is things per amount of liquid (solvent)
    so the calculation formula for molarity is M = mol/L
    that means Molarity = moles over Volume in liters
  6. Feb 13, 2009 #5
    so your equation would be M = mol/L, but you have it in Kg. This is not a problem because one gram of water is approximately equal to one ml of water. So if you have one L of water, that would mean that 27% of it is H2SO4. In one liter of water there are 1000 ml of water, so that means that 27% of 1000 is 270. That means that there are 270 ml of H4SO4 (aq). There are 1.198g/ml, so there are 323.46 grams of H2SO4. The molar mass of H2SO4 is 98 grams per mole, so to get moles of H2SO4, you divide the grams by the molar mass. That means that there are 323.46/98 = 3.3 moles of H2SO4. Now you do the rest, so i dont tell you the whole thing. =D
  7. Feb 14, 2009 #6


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    Staff: Mentor

    Question asks for molality, not molarity. Thus both your posts are unfortunately off.

    Additionally, it is 27% w/w solution, so 1000 mL of solution doesn't contain 270 mL of sulfuric acid, but 1 kg of solution contains 270 g of sulfuric acid. That is huge difference.
  8. Feb 14, 2009 #7
    oh i thought it was a spelling mistake :tongue2:
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