Question

Naturally occurring silicon consists of 92.21% 28Si, which has a mass of 27.97693 amu, 4.70% 29Si, which has a mass of 28.97649 amu, and 3.09% of 30Si, which has a mass of 29.97376 amu. Calculate the atomic mass of silicon

Answer 1

First convert percentage abundance to fractional abundance

92.21% = 92.21/100 = 0 .9221

4.70% = 4.7/100 = 0. 047

3.09% = 3.09/100 = 0.0309

Now multiply the isotopic mass by the fractional abundance to find the portion of the atomic mass from each isotope

28Si (27.97693amu * 0.9221) = 25.7975amu

29Si (28.97649amu * 0.0470) = 1.3619amu

30Si (29.97376amu * 0.0309) = 0.92619amu

Now add the isotopic portions to find the atomic mass.

25.7975amu + 1.3619amu + 0.92619amu = 28.0856amu

The atomic mass of silicon = 28.0856amu