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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.


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

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林宜悉 發佈於 2020 年 3 月 28 日
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