Answer:
start by getting its isotpic mass. you can do this by taking the mass of an atom and dividing it by 12.01, which is kinown as the carbon standard and is used in the calculation for any element. this will give you a ratio. multiply this number by 12 amu and it will give you the isotopic mass. Next, multiply this number by the percentage in abundance as a fraction (ex. 92.12% = .09212)..... the resulting number will be your atomic mass ONLY for that isotope. To calculate the ATOMIC MASS of an element, you must repeat the previous steps for ALL isotopes. When this is finished, add the atomic masses for the isotopes together the get the atomic mass for that element. For a visual, we'll use Si (28)
(mass of Si atom)/ (mass of C standard) = 2.331411 that's your ratio
2.331411 x 12 amu = 27.97693 amu .... that's your isotopic mass for that specific isotope
27.97693 x .9223 (or 92.23%) = 25.8031..... that's your atomic mass for that isotope
Now the similar calculations done with the other isotopes of silicon give us..
Si(29) as (28.976495x.0467=1.3532) and Si(30) as (29.973770x.0310=.9292)
Lastly, add those together to get...
25.8031 + 1.3532+ .9292 = 28.09 amu which is the atomic mass for the element Silicon.
Good luck, chemistry sucks.