J.R. S. answered 01/13/21
Ph.D. University Professor with 10+ years Tutoring Experience
2K + CaF2 ==> 2KF + Ca
Assuming K is present in excess...
10 g CaF2 x 1 mol CaF2 / 78.1 g x 2 mol KF / mol CaF2 x 58.1 g KF/mol KF = 14.9 g KF theoretical yield
% yield = actual/theoretical (x100) = 12.2 g / 14.9 g (x100%) = 81.9% = 80% (1 sig. fig.)
(NOTE: the values given in the problem have 3 sig. figs. for 12.2 g and only 1 sig. fig. for 10 g CaF2, so to give an answer of 81.98% to 4 sig. figs. is incorrect.)