 Okay, so let's continue our discussion of average atomic mass with this next problem where instead of looking for the actual average atomic mass, we're looking for one of the isotopes. So we're going to have to use that algebraic equation to manipulate it, okay? So it says copper has two naturally occurring isotopes. Copper 63 has a mass of 62.939 AMU and a relative abundance of 69.17%. The atomic weight of copper, or use the atomic weight of copper to determine the mass of the other copper isotopes. So in order to do this problem, you have to have a periodic table unless you've memorized the atomic weight of copper, okay? So the atomic weight of copper is 63.5 AMU. Notice also it didn't give us the abundance of isotope 2, okay? What are we really looking for? We're looking for the mass of isotope 2 and we couldn't find that if we didn't know the abundance. So how do we do that first? So we're going to have to look at the abundance of the other one because it says there's only two naturally occurring, right? So that means this one, was this one, have to be equal to 100%, right? Yes, exactly. So we take 100 and we subtract that, or we subtract that other isotope, 69.17. So 100% minus 69.17% and we get 30.03%, sorry, percent. Okay, is everybody okay with how to get that? Now if you recall, so this is what we're looking for, so if you recall the formula for average atomic weight, okay? So make sure you're able to remember that for these problems, okay? If you don't do that, we'll be able to do that, okay? At this point in time, we could go ahead and rearrange it all just to solve for the mass of the second isotope, but oftentimes, I have recorded videos of those already. Let's do one where we just plug it in and then just move everything around, algebraic, okay? So notice here it says the abundance of isotope one, but this is the ratio of isotope one. So in order to get the ratio from the abundance, we have to divide by 100%, okay? Canceling out of our percentages and we get 0.6917. Do the same thing for this one, 0.30830, is everybody okay with what I've just done? Okay, the atomic weight of copper, that's the atomic weight there, okay? So now I'm just going to put everything into the formula, okay? Any questions so far, I guess I should say. Now I'm going to again use my units to remind me that in the end, right, this mass is going to be an AMU, okay? That's a mass here. Ratio of isotope one, 0.6917 times the mass, 62.939 AMU plus the other ratio, 0.3083 times, we don't know, massive isotope 2. Okay, that's what we're looking for. So again, like I said, what I'm going to do for you guys today is just show how I would think about it if I were trying to figure this out, okay? So the first thing I would do is multiply these two things together, okay? So I get 0.6917 times 62.939, okay? And I get 43.53 something, okay? With that, right, that 43.53, this is added to this other. This is like A plus B, right? So I can subtract this from this if we remember our algebraic rules, right? So what I'm going to do is take that 43.53 and subtract it from 63.55. So when I do that, I get 20.02 AMU approximate, okay? And then I still have this equal to 0.3083 times massive isotope, okay? But at this point, I could just be like, I don't need these parentheses anymore, okay? So this times this, well, if I want to get rid of this, I'm going to divide both sides by it, right? So now I'm going to just say 0.3083. And then of course, if I do the same here, that just gets rid of it. So that's going to give me, this number is going to be the mass of isotope, too. Is everybody okay with what I'm saying? So the mass of isotope, 2, is going to be 20.015, whatever, divided by 0.3083. And hopefully I get the same number as you guys, 64.92 AMU. Okay, so why did I only take it to 4? Because of significant digits, okay? So that's the mass of the other isotope. Any questions on how we do that? Yes, ma'am. You well remember I gave you periodic table? No. Yeah. So it's on the periodic table? Yeah. So I'll always go back to the periodic table and look for that atomic weight, okay? Sometimes the problem's asking, what is the average atomic weight? And you just go look at the periodic table and be like, okay, I know I need to show my work, but I know what the answer I need to get to is, okay? Yeah. Yeah, that's because I carried all my digits through and you cut off at everything. Okay? I would give it to you. It's the same answer.