Finding the atomic mass of a hypothetical element.

[Aeropsia]

Here is a problem I have. Not really sure where to start. I'm not looking for the answer, I just want someone to point me in the right direction.

A hypothetical element X has three isotopes: 40X, 41X, and 42X. Their abundances are 72.00%, 9.00% and 19.0% respectively. What is the atomic mass of X?

(A) 40.5 amu (B) 40.8 amu (C) 41.0 amu (D) 41.5 amu

Never mind, found info on how to find the answer here, should have Googled first... http://chemistry.about.com/od/chemistryquickreview/a/atomicweight.htm

 

[Nino MMP]

To find the atomic mass of X you need to calculate the relative atomic mass (A) for each isotope and then multiply it by its abundance. Then you can add up all the A x % values to get the average atomic mass of X. For example, A for 40X is 40.0 amu, for 41X it is 41.0 amu and for 42X it is 42.0 amu. Therefore, the atomic mass of X is (40.0 x 72.00%) + (41.0 x 9.00%) + (42.0 x 19.0%) = 40.8 amu, which means the answer is (B) 40.8 amu.