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Homework Help: Molar concentration

  1. Feb 25, 2010 #1
    1. The problem statement, all variables and given/known data
    A sample of air was collected to assess the safety level of nitrogen trichloride in the air around a swimming pool. It was found that the air contained 0.0025% by mass of nitrogen trichloride. Calculate the molar concentration of nitrogen trichloride in the sample collected.

    2. Relevant equations

    3. The attempt at a solution
    I know that mole is directly proportional to volume for gas. But it's percentage by mass. I have no idea how to do this.
    The answer is 2.07x10^-4 moldm^-3 .

    Can anyone explain to me or give me some clues?

    I know that air is a mixture and should be done differently from finding empirical formula of a compound.

    Thank you
  2. jcsd
  3. Feb 25, 2010 #2
    I worked through it and got 2.65 * 10^-7 M, a bit off from the answer you gave. I assumed standard temperature and pressure, are you given different information in the problem?

    I started by calculating the mass of 1 L of air at STP, then used % by mass to calculate mass of the NCl3, used that mass + molecular weight to find the # of moles in one liter.

    I'm kinda curious to see why my answer is different and if anyone else solved the problem in a different way.
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