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In this video, I'm going to explain how to solve problems with average atomic mass.

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In the first one, we know the abundance of the natural isotopes of one element and their atomic masses.

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So we have this element. We don't know the element. That doesn't matter.

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And we know that it has three isotopes.

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And these numbers here are the abundance of each one of these isotopes.

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and then we also need to know the atomic mass of each one of these isotopes that

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are these numbers okay so to calculate the average atomic mass we will need to

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multiply the abundance of each isotope by its atomic mass so this is the first

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abundance in the first atomic mass, then the second isotope's abundance times its

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atomic mass, and we still have another stable isotope and its mass.

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Okay, so check. We have multiplied each abundance by its atomic mass, and we divide

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everything by 100. So right now we need a calculator, we do these numbers in the

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calculator, it's easy, and the average atomic mass of this element, we don't

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know which element is, it's not important, is 20.17 atomic mass units. In these

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problems we can use also this formula to calculate the abundance of an element,

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the isotopes of an element, if we know the average atomic mass, like in this case.

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Lithium has an elemental atomic mass of 6.941.

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This is the average atomic mass of the element.

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So it's this information here. So in this case we know this information.

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And it has two natural occurring isotopes.

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we are given or we know the masses of each one of these isotopes but we don't

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know the abundance of each one. So how do we do this? We are going to

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use the same formula only that in this case we know that the average atomic

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mass is 6.941 equals and then the abundance of each isotope we don't know

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so I'm going to say that the abundance of the isotope 6.0151 is

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for example X percent okay so I'm going to call this X because I don't know how

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much is that and then for the second isotope the mass is this number it will

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be like Y percent but we know that X and Y as there are only two isotopes the

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100 percent so Y is 100 minus X so I'm going to use this in this part so six

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7 times 100 minus X is the abundance of the second isotope so now I need to

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work out this equation to solve this equation for X so I'm the

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100 cos multiplying here I'm going to multiply this to remove the brackets

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this number multiplied by 100 is this number this number I'm going to move it

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here so i isolate the x the thing that we don't know i do this subtraction it's easy with my

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calculator there is a mistake here sorry this goes here so i have in this minus 7.5 equals

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now i do this and the number is minus 1.001 x okay so x is minus 7.5 divided by minus 1.001

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the number is 7.49 remember that it is percent that's the abundance of this isotope okay

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the isotope that has an average atomic mass of more or less six and then the abundance of the

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other isotope is 100 minus x and we can see that this isotope is much more abundant than

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the isotope with the lithium-7 than lithium-6 and you can notice that the average atomic mass

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is more like as a number that is closer to seven that it is the six so it's it's normal that

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lithium seven is much more abundant than lithium six i hope this helps see you in class