yego.me
💡 Stop wasting time. Read Youtube instead of watch. Download Chrome Extension

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


2m read
·Nov 10, 2024

So let's say that we have some mystery substance here, and we know that it's a pure element. 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 could take a sample of a substance and think about the various atomic masses of the different isotopes in that substance. 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. This is telling us that this looks like maybe—I don't know—let's call this 82 percent of our sample has an atomic mass of 88 universal atomic mass units. About this looks like about 7 percent of our sample has an atomic mass of 87 universal atomic mass units. It looks like 10 percent has an atomic mass of 86 universal atomic mass units, and it looks like about 1 percent of our sample has an atomic mass of 84 universal atomic mass units.

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 0.1 times 86 plus unless it should add up to 100 percent. This is 89, and then this gets us to 99. So then, another 1 percent, 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 even if we did 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.

More Articles

View All
Morgan Freeman Decodes the Mark of the Beast | The Story of God
I’ve come to the biblioteca Kasana 10z near the Vatican to meet an expert in early Christianity. Campaigns it–some she tells me she has compelling evidence that reveals the identity of the beast. [Music] Good morning, so what exactly do we have here? …
Finding perimeter when a side length is missing | Math | 3rd grade | Khan Academy
What is the perimeter of the figure below? So down here we have this figure and we are asked to find the perimeter of this figure. Perimeter is the distance all the way around the outside of a shape. So in this case, if I were to walk around the outside…
Introducing Khanmigo for teachers
This is Conmigo, an AI-powered guide designed to help all students learn when subjects are giving them trouble. Conmigo can help. Kamika was fun and can transform learning into an adventure. Kanmigo is not just for students; teachers can use it too by tog…
Experiencing the Rich History and Food of Canada's Season of Bounty | National Geographic
Iconic! I love to know the life of the stuff I’m going to eat before it shows up on my plate. That’s why I’m in Canada’s Maritime Provinces and Quebec during the season of bounty. To connect with the land and rich history through people and food. Good mo…
That Time I Said Prime Numbers For 3 Hours
Have you seen the video where I count by prime numbers for 3 hours? Well, that video is take two. Yeah, here’s what happened. The first time I did it, the guy whose job was to stop me after 3 hours read the clock wrong, and I was stopped after 2 hours and…
Why I'm NOT Investing in Bitcoin! | Shark Tank's Kevin O'Leary & Anthony Pompliano
You you and I originally clashed, if you want to call it that, around a topic that you’re so engrained with. It’s part of your brand; it’s bitcoin. I’m like everybody else saying, “If it works, I should own some,” but frankly all I’ve seen so far is volat…