Question

Calculate the atomic mass of silicon. The three silicon isotopes have the following atomic masses and % abundances...(27.9769 amu and 92.2297%), (29.9765 amu and 4.6832%), (29.9738 amu and 3.0872%)

Answer 1

The calculated atomic mass of silicon is 28.104 u.

The average atomic mass of Si is the weighted average of the atomic masses of its isotopes.

We multiply the atomic mass of each isotope by a number representing its relative importance (i.e., its % abundance).  

Thus,  

0.922 97    × 27.9769 u = 25.821 8      u

0.046 832 × 28.9765 u =   1.357 03    u

0.030 872 × 29.9738 u =   0.925 351 u

                         TOTAL = 28.104         u

Note: Your percentages don’t add up to 100.0000 % (I think the first one is incorrect).