Percent Composition Of the Isotopes of an Element

Percent Composition

Of the Isotopes of an Element

Percent CompositionAt the conclusion of our time together, you should be able to:

Determine the percent composition of 2 isotopes of an element if the average atomic mass and the mass of each isotope is given.

Remember How to Determine Average Atomic Mass

1. Determine Relative Abundance –

% abundance divided by 100

2. Determine mass of each isotope and multiply relative abundance by this mass

3. (keep all digits your calculator gives you)

4. Determine Average Atomic Mass by adding up all the individual masses, round to “2” decimal places

(0.98 x 1 amu) + (0.01 x 2 amu) + (0.01 x 3 amu)

1.03 amu

98 1 1

But What If…

We know the average atomic mass and the mass of each isotope but not the percentage of each???

How to Determine Percent AbundanceHow to Determine Percent Abundance

Use the following equation:Use the following equation:

Ave. Atomic Mass = (X)(Mass of 1Ave. Atomic Mass = (X)(Mass of 1stst isotope) isotope)

+(1-X)(Mass of 2+(1-X)(Mass of 2ndnd isotope) isotope)

1.1. Solve for “X”Solve for “X”

2.2. Answer will be a decimalAnswer will be a decimal

3.3. Convert the answer to a percentageConvert the answer to a percentage

4.4. Subtract this answer from 100 % for the other Subtract this answer from 100 % for the other answer.answer.

5.5. The sum of the two answers should equal 100 %The sum of the two answers should equal 100 %

2) Lithium exists as 2 isotopes, Li-6 and Li-7. Calculate the % abundance for each isotope of lithium.

% Abundance Practice% Abundance Practice

AnswerAnswer

6.941 = (x)(6) + (1-x)(7)

= 6x + 7 – 7x

= -1x + 7

x = .059

Li-6 is 5.9% abundant

Li-7 is 94.1% abundant

When I Went for

my Physical

this Summer…

My Doc asked about my physical activity the day before…

I told him that I waded around a lake, climbed a mountain, ran from a snake, and got poison ivy.

Inspired by my story, my doc said, "You must be an awesome outdoors

man!“

"No," I replied, "I'm just a terrible golfer."

% Abundance Practice% Abundance Practice

1) Boron, B, exists as 2 isotopes, B-10

and B-11. Calculate the % abundance for each isotope of Boron.

AnswerAnswer

10.811 = (x)(10) + (1-x)(11)

= 10 x + 11 – 11 x

= -1 x + 11

x = 0.189

B-10 is 18.9 % abundant

B-11 is 81.1 % abundant

Percent CompositionPercent CompositionLet’s see if you can:Let’s see if you can:

Determine the percent composition of 2 isotopes of an element if the average atomic mass and the mass of each isotope is given.

When the teen asked her parents what they were doing they replied…”None of your

Bismuth” - Bi

Copper consists of two naturally occurring isotopes with masses of 63 and 65 amu. The average atomic mass of Cu is 63.546 amu. Calculate the abundance of each isotope.

1.1. 6363Cu = 40.3% Cu = 40.3% 6565Cu = 59.7%Cu = 59.7%

2.2. 6363Cu = 69.7% Cu = 69.7% 6565Cu = 30.3%Cu = 30.3%

3.3. 6363Cu = 29.7% Cu = 29.7% 6565Cu = 70.3%Cu = 70.3%

4.4. 6363Cu = 31.3% Cu = 31.3% 6565Cu = 68.7%Cu = 68.7%

5.5. 6363Cu = 72.7% Cu = 72.7% 6565Cu = 27.3%Cu = 27.3%1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

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