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- [Instructor] So let's say that we have
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some mystery substance here,
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and we know that it's a pure element,
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and we need to figure out what it is.
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Well, scientists have a method,
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and we go into the details,
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or more details, in other videos,
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called mass, sometimes it's known as mass spectrometry
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or mass spectroscopy.
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It's a technique where you can take a sample of a substance
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and think about the various atomic masses
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of the different isotopes in that substance.
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And that's what we have right over here.
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They tell us the mass spectrum for an average sample
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of a pure element is shown below.
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So let's say it's this pure element.
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So what this is telling us is,
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this looks like maybe, I don't know,
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let's call this 82% of our sample
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has an atomic mass of 88 universal atomic mass units.
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About, this looks like about 7% of our sample
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has an atomic mass of 87 universal atomic mass units.
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It looks like 10% has an atomic mass
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of 86 universal atomic mass units,
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and it looks like about 1% of our sample
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has an atomic mass of 84 universal atomic mass units.
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And so from this information,
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we can try to estimate what the average atomic mass
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of this mystery element is.
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We could calculate it as 0.82 times 88,
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plus, let's call this 7%,
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so 0.07 times 87,
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plus 10%, 0.1, times 86,
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plus, let's see, it should add up to 100%.
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This is 89,
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and then this gets us to 99,
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so then another 1%,
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0.01 times 84.
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And so if we were to do this calculation,
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this is our estimate
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of the average atomic mass of this element.
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We could type this into a calculator
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and get some number
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and then look that up on a periodic table of elements,
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or we could just try to estimate it.
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We can see that it's going to be close to 88
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because that's where the highest percentage is.
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When we're taking the weighted average,
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we have the highest weight right over there.
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But these other isotopes,
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these other versions of the element
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that have a different number of neutrons,
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which changes its atomic mass,
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they're going to bring the average down.
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So our average atomic mass
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is going to be a little bit less than 88.
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So let's look up a periodic table of elements.
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What element here has an atomic mass
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a little bit less than 88?
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Well, yttrium is 88.91,
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but we know it can't be that because none of the isotopes
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have an atomic mass above 88.
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So we can rule out yttrium.
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Strontium is looking pretty good.
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It's exactly what we predicted,
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a little bit less than 88,
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and rubidium is a lot less than 88.
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So our, even if we were to do the calculation,
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we could feel confident
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we're not going to be as low as rubidium.
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So I'm feeling very confident just eyeballing it,
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just estimating,
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this is going to be a little bit,
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have an average atomic mass a little bit less than 88,
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which tells me that this is strontium.
