Posted by **Alison** on Monday, August 29, 2011 at 9:15pm.

A certain element has four isotopes.

5.85% of has a mass of 53.93961 .

91.8% of has a mass of 55.934939 .

2.12% of has a mass of 56.93539 .

0.282% of has a mass of 57.93328 .

What is the average atomic mass of element ?

Express your answer numerically to three significant figures.

- Calculating Averages? -
**Alison**, Monday, August 29, 2011 at 9:17pm
i got 56.2amu but it was wrong. i don't know how to find the average amu?

- Calculating Averages? -
**TutorCat**, Monday, August 29, 2011 at 9:34pm
http://www.algebralab.org/practice/practice.aspx?file=algebra_averageatomicmass.xml

- Calculating Averages? -
**Reiny**, Monday, August 29, 2011 at 9:59pm
I would suggest finding the 100% mass of each of the isotopes, adding those, then dividing by 4

I will find the 100% mass for the first one:

if 5.85% has a mass of 53.93961, then

100% has a mass of 53.93961/5.85*100 or 922.0446154

I could be way off base here, perhaps a chemist could enlighten us.

The webpage suggested by "TutorCat" makes sense to me.

- Calculating Averages? -
**Kevin**, Tuesday, August 30, 2011 at 2:23am
just throwing out a number because i'm bored: 55.8741 was what i got. I'm no chemist, but i wanted to take a shot at it and now i'm curious how close the real answer is to mine.

- Calculating Averages? -
**A**, Friday, September 23, 2011 at 6:56pm
55.8

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