Question

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Oxygen has three isotopes: O16 , O17 , and O18 . In any given sample, 99.757% is O16 , 0.038% is O17 , and 0.205% is O18 . The atomic mass of each is as follows: O16=15.995 amu ; O17=16.999 amu ; O18=17.999 amu .

What is the average atomic mass of oxygen?

A) 15.995 amu

B) 15.999 amu

C) 16.060 amu

D) 16.099 amu

Answer 1

Answer:

15.999amu

Explanation:

There is way more O16 than any other, so O17 and O18 affect it very little.

Answer 2

Answer:

The answer is 15.999 amu

Explanation:

Data:

mO16 = 15.995 amu

mO17 = 16.999 amu

mO18 = 17.999 amu

%O16 = 99.757%

%O17 = 0.038%

%O18 = 0.205%

The molecular mass of oxygen is going to be equal to:

$mO = \frac{(mO16xpercentO16)+(mO17xpercentO17)+(mO18xpercentO18)}{100}$

replacing, we have:

$mO = \frac{(15.995x99.757)+(16.999x0.038)+(17.999x0.205)}{100} = 15.999489 amu$

this is approximately 15.999 amu