Isotope Mass(amu) abundance 28Si 27.98 (???????) 29Si (??????) 4.70% 32Si 29.97

Question

Isotope                                  Mass(amu)                             abundance 28Si                                       27.98                                       (???????) 29Si                                      (??????)                                    4.70% 32Si                                        29.97                                      3.09% How do i get the missing information? Isotope                                  Mass(amu)                             abundance 28Si                                       27.98                                       (???????) 29Si                                      (??????)                                    4.70% 32Si                                        29.97                                      3.09% How do i get the missing information?

Explanation / Answer

are you sure that isn't 30 Si and not 32 Si? 32 Si would not havean amu of 29.97. Also, 32 Si is synthetic: http://en.wikipedia.org/wiki/Silicon (ctrl+F, "isotope") Anyway, we know the abundance must sum to 100, so abundance of 28 Si = 100 - 4.7 - 3.09 = 92.21% We also know that the weighted-average atomic weight of Si is28.0855 So 28.0855 = 0.9221*27.98 + 0.047*x + 0.0309*29.97 x = 28.916

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