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	Chemistry I had a question on our quiz for chemistry that I could not answer. The teacher did not tell us how abundance (percent) is found in Atomic mass. She only showed us how to get the amu from mass (amu) x abundance (%). Question: How would you find abundance (%) with the isotope (Icant write small numbers!) 35CL Mass - 34.97? thank you for helping me before :) Cathy
	1 Answers Asked By Cathy

	The atomic mass that appears in the periodic table is the weighted average of the mass of all the naturally occuring isotopes. In the case of Chlorine, the atomic number is 17. For the sake of this problem, I'm assuming that there are just two isotopes...one with 18 neutrons and one with 17 neutrons. (One would need more information than what you provided to answer the question if there were more than two isotopes.) That gives us isotopes with mass 35 (17protons + 18 neutrons) and 34 (17p + 17 n). To get a weighted average of 34.97 we need to figure out what proportion of the total supply in nature (100%) is in the 35 amu state and how much is in the 34 amu state. The math is as such: 34.97 * 100% = [35 * x] + [34 *(100 - x)] Solving for x gives you the % of isotope occuring in the 35 amu form. Note that since the answer is 34.97, we should expect a very high % of isotopes to be in the 35 amu form. I hope this helps. If this explanation did not clarify things for you, please "submit an answer" with more information. =) Answered By Aprenda, Inc.

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