 ## Subtitles section Play video

• - [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.

• 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

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B1 atomic element average sample substance estimate

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

• 2 0
林宜悉 posted on 2020/03/27
Video vocabulary