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Homework Help: Molecules from concentration

  1. Oct 5, 2007 #1
    Hey, just wondering if i can get some verification with my answers.
    I need to find the number of molecules from a concentration that is 1.0x10^-22 M of AsCl3 in one mL.

    I divided my concentration by 1000mL to get it down to a concentration per mL and multiplied by avogrado's number. My answer is 0.06. However, I'm not sure that's right as the unit should be an integer.

    thoughts? i think I'm missing something.
     
  2. jcsd
  3. Oct 6, 2007 #2
    Hello jen333,

    You raise an interesting question.

    Your calculation is right. The answer is approximately 0.06 molecules of AsCl3. Ofcourse 0.06 molecuels cannot exist , remember you are working with approximate values here. You can safely say the number is negligible.

    If in 1000 ml there are 60 molecules of AsCl3 , then if we take 1 ml of this solution there exists a number of AsCl3 molecules in solution that , statistically speaking, tends towards zero. Ofcourse this might not make sense to you , but in a problem like this , matter cannot be treated as "bulk" but individual particles must be studied. That is , the distribution of such a small number of molecules in a considerably large volume cannot be determined by taking 'average values'. In reality the presence of 0 , 1, 2 ,... or 60 molecules of AsCl3 in 1 ml solution is purely random.
     
  4. Oct 6, 2007 #3
    your calculations look correct but it means that there is much less than 1 molecule per mL. Its the volume of 1mL that gives a number less than 1. The starting concentration is so dilute that it means you would need (1/.06) mL = 16.7 mL to find a molecule of AsCl3
     
  5. Oct 7, 2007 #4
    [tex] C = 1*10^{-22}Mdm^{-3} [/tex] ?
     
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