J.R. S. answered 09/23/21
Ph.D. University Professor with 10+ years Tutoring Experience
4Al(s) + 3O2(g) ==> 2Al2O3(s) ... balanced equation
Assuming O2 is not limiting, we have the following:
Theoretical yield of Al2O3 = 20.0 g Al x 1 mol Al / 27.0 g x 2 mol Al2O3 / 4 mol Al x 102 g/mol = 37.8 g Al2O3
% yield = actual yield / theoretical yield (x100%)
32.7 g / 37.8 g (x100%) = 86.5% yield