KlausZahn Posted December 8, 2018 Share Posted December 8, 2018 In my chemistry book there is the task that I should calculate the "average of atomic mass unit" of two isotopes of chlorine. The solution is that the "atomic mass units" are calculated. Then they are added together. (Example: 26,496u + 8,957u = 35,453u). And that is already the result. But if you calculate the average, you still have to divide by two. Why I dont have to divide in this calculation? Thanks for your answers Link to comment Share on other sites More sharing options...
studiot Posted December 8, 2018 Share Posted December 8, 2018 20 minutes ago, KlausZahn said: In my chemistry book there is the task that I should calculate the "average of atomic mass unit" of two isotopes of chlorine. The solution is that the "atomic mass units" are calculated. Then they are added together. (Example: 26,496u + 8,957u = 35,453u). And that is already the result. But if you calculate the average, you still have to divide by two. Why I dont have to divide in this calculation? Thanks for your answers Can you post the question? 26 and 8 are not isotpic masses for chlorine or any element. Nor does any element have 26 protons and only 8 neutrons, or the other way round. Link to comment Share on other sites More sharing options...
KlausZahn Posted December 8, 2018 Author Share Posted December 8, 2018 34 minutes ago, studiot said: Can you post the question? 26 and 8 are not isotpic masses for chlorine or any element. Nor does any element have 26 protons and only 8 neutrons, or the other way round. The question is in german. I have to calculate the medium atomic mass of natural chlorine. The isotops are Cl-37 (24,23%) and Cl-35 (75,55%) Link to comment Share on other sites More sharing options...
Sensei Posted December 8, 2018 Share Posted December 8, 2018 (edited) 2 hours ago, KlausZahn said: But if you calculate the average, you still have to divide by two. Why I dont have to divide in this calculation? In mathematics, if you calculate average from some numbers, you indeed add them together, then divide by their quantity: ( x1 + x2 + ... xn ) / n But in the case of calculation of "average Chlorine mass" it's not the case, as you use percentages of abundance. They sum to one (100%). 0.7577 + 0.2423 = 1 If you would be able to count the all atoms to the single one, you could get such equation: ( 34.969u * 7577 + 36.966u * 2423 ) / 10000 (if you would have 10 thousands Chlorine atoms) Edited December 8, 2018 by Sensei Link to comment Share on other sites More sharing options...
KlausZahn Posted December 8, 2018 Author Share Posted December 8, 2018 ok, thank you Link to comment Share on other sites More sharing options...
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