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Some Basic Concepts of Chemistry
Question
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An element, X has the following isotopic composition;


200X: 90%

199X : 8.0%

202X ; 2.0 %

The weighted average atomic mass of the naturally -occurring element X is closet to:

  • 200 amu

  • 201 amu

  • 202 amu

  • 199 amu

Answer

A.

200 amu

Weight of  200X = 0.90 x 200 = 180.00 amu
Weight of 199X  = 8.08 x 199 = 15.92 amu
Weight 0f  202X =  0.02 x 202 = 4.04 amu
Total weight = 199.06 = 200 amu

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