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  • - [Instructor] So let's say that we have

  • some mystery substance here,

  • and we know that it's a pure element,

  • and we need to figure out what it is.

  • Well, scientists have a method,

  • and we go into the details,

  • or more details, in other videos,

  • called mass, sometimes it's known as mass spectrometry

  • or mass spectroscopy.

  • It's a technique where you can take a sample of a substance

  • and think about the various atomic masses

  • of the different isotopes in that substance.

  • And that's what we have right over here.

  • They tell us the mass spectrum for an average sample

  • of a pure element is shown below.

  • So let's say it's this pure element.

  • So what this is telling us is,

  • this looks like maybe, I don't know,

  • let's call this 82% of our sample

  • has an atomic mass of 88 universal atomic mass units.

  • About, this looks like about 7% of our sample

  • has an atomic mass of 87 universal atomic mass units.

  • It looks like 10% has an atomic mass

  • of 86 universal atomic mass units,

  • and it looks like about 1% of our sample

  • has an atomic mass of 84 universal atomic mass units.

  • And so from this information,

  • we can try to estimate what the average atomic mass

  • of this mystery element is.

  • We could calculate it as 0.82 times 88,

  • plus, let's call this 7%,

  • so 0.07 times 87,

  • plus 10%, 0.1, times 86,

  • plus, let's see, it should add up to 100%.

  • This is 89,

  • and then this gets us to 99,

  • so then another 1%,

  • 0.01 times 84.

  • And so if we were to do this calculation,

  • this is our estimate

  • of the average atomic mass of this element.

  • We could type this into a calculator

  • and get some number

  • and then look that up on a periodic table of elements,

  • or we could just try to estimate it.

  • We can see that it's going to be close to 88

  • because that's where the highest percentage is.

  • When we're taking the weighted average,

  • we have the highest weight right over there.

  • But these other isotopes,

  • these other versions of the element

  • that have a different number of neutrons,

  • which changes its atomic mass,

  • they're going to bring the average down.

  • So our average atomic mass

  • is going to be a little bit less than 88.

  • So let's look up a periodic table of elements.

  • What element here has an atomic mass

  • a little bit less than 88?

  • Well, yttrium is 88.91,

  • but we know it can't be that because none of the isotopes

  • have an atomic mass above 88.

  • So we can rule out yttrium.

  • Strontium is looking pretty good.

  • It's exactly what we predicted,

  • a little bit less than 88,

  • and rubidium is a lot less than 88.

  • So our, even if we were to do the calculation,

  • we could feel confident

  • we're not going to be as low as rubidium.

  • So I'm feeling very confident just eyeballing it,

  • just estimating,

  • this is going to be a little bit,

  • have an average atomic mass a little bit less than 88,

  • which tells me that this is strontium.

- [Instructor] So let's say that we have

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B1 atomic element average sample substance estimate

Worked example: Identifying an element from its mass spectrum | AP Chemistry | Khan Academy

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    林宜悉 posted on 2020/03/27
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