Suppose that a new element (E) were discovered that existed as 3 naturally occurring isotopes. 25.0% of the atoms had a mass of 278amu, 38.0% had a mass of 281 amu, and the remainder had a mass of 285amu. What would its atomic mass be?

Question

Suppose that a new element (E) were discovered that existed as 3 naturally occurring isotopes.  25.0% of the atoms had a mass of 278amu, 38.0% had a mass of 281 amu, and the remainder had a mass of 285amu.  What would its atomic mass be?

michele_laino · Answer

the atomic mass is:
$$0.25*278+0.38*281+0.27*285=...$$
please complete that computation

anonymous · Answer

keep in mind that if there are three isotopes, that means that all three of these isotopes add up to 100% of the element E.

Since it's known that you have one isotope with a 25% abundance and a second 38% in abundance, that would leave you with the abundance of the third...when you figure out the % abundance of the third isotope, you can then use the equation to find atomic mass... 

(%abundance of isotope1* mass of isotope1) + (%abundance of isotope2* mass of isotope2) + (%abundance of isotope3* mass of isotope3) = average amu

hope that helps