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Problem 104 Medium Difficulty

Although exact isotopic masses are known with great precision for most elements, we use the average mass of an element's atoms in most chemical calculations. Explain.

Answer

Example: Carbon exists as $_{6}^{12} \mathrm{C},^{13} \mathrm{C}$ and $_{6}^{14} \mathrm{C}$ . Any sample of carbon contains all these three isotopes. So, we need to use the average atomic mass of carbon whenever it is necessary instead of its atomic mass. It is 12.01 amu.

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Stephanie C.

University of Central Florida

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Video Transcript

Oh, so in this question, we're talking about isotopes. And how come when we talk in atomic mass we don't use, You know, um, the exact mass of the compound bones dead use, like other, another value that I'll talk about later. And even though we know the master of each Adam of any particular compounds wearable to know that mass. But that's not the mass that is the atomic mass. So it's all about, um, isotopes and relative abundance. So we're gonna talk. Let's talk about I'm carpet, for example. But, yes, the reason for this is relative abundance may just write that down. So what this means is the in any particular sample off And Adam, there is a relative abundance of each kind of isotope. Great. So each, uh, isotope sorry has its own, um, atomic mass, which I'll call a m atomic mess. Yeah, uh, you're right. That properly? Yes. Because what basically and I stopped is to refresh my memory is compounds with the same number of protons, but different number off, um, neutrons. Number of neutrons is different. So therefore the whole atomic mass that Adam would be different. So we have carbon 12 12 being it's wait, it's atomic mass. We have carbon 13 and also carbon 14. Our planet. This is an atomic mass units relatively so carbon 12 ways. Exactly 12 atomic mass units. So it's carbon 13 is well, it's Corbyn 14. Yet the atomic mass of carbon on the product table is 12.17 So what is happening is actually quite a large Dustin, 12.0 seven. All right, So the reason for this again is relative abundance. Because in nature, when we're talking about carbon 12 we have it in the 98.9% abundance and carpet 13 we have in a 1.1 percent abundance, and carbon 14 is a very negligible number. Extremely tiny amount of carbon 14 on our planet. So that means if we scoop up, you know, carbon arbitrarily in our hands 98.9 of those atoms would weigh 12 atomic capacity is what 1.1 of those atoms would rate 13 atomic mass. Ian's. So then they actually would it make sense for us to just say that this is the weight of carbon. So So what we do is we add up 98.9% of 12 plus 1.1% of three teen, and that then gives us this number, which then accounts for each isotope, so that's where this number comes from.

McMaster University
Top Chemistry 101 Educators
Stephanie C.

University of Central Florida

Nadia L.

Rice University

Allea C.

University of Maryland - University College

Morgan S.

University of Kentucky