chemistry help?

Question

calculate mass of silicon using the following exact atomic masses and % abundance for each of it's naturally occuring isotopes? ^28Si(92.21%), atomic mass of 27.97693 amu, ^29Si(4.70% atomic mass of 28.97649 amu, and ^30Si(3.09) atomic mass of 29.97376 amu.
please explain how you could the answer thank you so much

Answer 1

When you answer, work out using all the decimal places. I will do it with round numbers.

Thus you have 92% of Si28, so add 92% or (0.9221 X 27.97...) to 4.7% or (0.047 X 28.97... )etc.

that will give you the accurate mass

Answer 2

This is just a weighted mean.

0.9221 x exact atomic mass of 28Si + 0.0470 x exact atomic mass of 29Si + ...